Person A wrongs person B, the case ends up it courts and then, for a whole different reason and in a whole different scenario, B wrongs A in a way which is worthy to make the case end up in court as well. Can it possibly be that another and indipendent case from the first one will be opened up in court or will it influence the reputation and the fate of B winning the first case in which he's a victim? I mean, can these scenarios merge in a single case? Jurisdictions: U.S, Belgium and Italy. Feel free not to answer for all the 3 jurisdictions, and it would be a pleasure if three different users answer for each jurisdiction.
In the United States, the answer is that B would probably get to decide whether to litigate the new claims separately or as part of the first case.
When someone A sues B, and B sues A back, B is bringing what we call a "counterclaim." In federal courts, this situation is governed by Rule 13 of the Federal Rules of Civl Procedure (the rules are roughly the same in most state courts):
(a) Compulsory Counterclaim.
(1) In General. A pleading must state as a counterclaim any claim that—at the time of its service—the pleader has against an opposing party if the claim:
(A) arises out of the transaction or occurrence that is the subject matter of the opposing party's claim; and
(B) does not require adding another party over whom the court cannot acquire jurisdiction.
(2) Exceptions. The pleader need not state the claim if:
(A) when the action was commenced, the claim was the subject of another pending action; or
(B) the opposing party sued on its claim by attachment or other process that did not establish personal jurisdiction over the pleader on that claim, and the pleader does not assert any counterclaim under this rule.
(b) Permissive Counterclaim. A pleading may state as a counterclaim against an opposing party any claim that is not compulsory.
The generally rule, then, is that if A's claims and B's counterclaim are about the same issue, B is required to file a counterclaim. These are called "compulsory counterclaims." Besides these, B is also "permitted" to bring claims that are not about the same issue. These are called "permissive counterclaims."
You're asking about a situation where a counterclaim arises "for a whole different reason and in a whole different scenario." In that case, the counterclaim would be considered permissive, so most jurisdictions would allow -- but not require -- B to present it as part of the litigation that A filed.
This means that the two lawsuits could become a single lawsuit, or they could be separate lawsuits. If they were maintained as separate lawsuits, the court should take steps to ensure that jurors in each case are unaware of the other, thus limiting the effect of either case on the other's outcome.
Some general principles without details or authority (which I will look up if I get a chance to cite chapter and verse):
In The U.S.
Person A wrongs person B, the case ends up it courts and then, for a whole different reason and in a whole different scenario, B wrongs A in a way which is worthy to make the case end up in court as well.
Can it possibly be that another and independent case from the first one will be opened up in court or will it influence the reputation and the fate of B winning the first case in which he's a victim?
When A sues B, and B has unrelated claims against A, then in common law jurisdictions, B has a choice. B can file unrelated counterclaims against A, or B can file a separate lawsuit. This risk involved in filing a separate lawsuit is that a later judge may decide that what looked like completely different facts to B actually involved a common nucleus of facts, in which case the separate lawsuit could be dismissed with prejudice if the first lawsuit produced a final judgment first.
If counterclaims are filed, any judgment for A is setoff against any judgment for B, resulting in a single net judgment.
If separate lawsuits are filed, and one produces a judgment for A against B and one produces a judgment for B against A, A can use the judgment against B as a credit to pay the judgment for B against A to the full extent possible. But, this would ordinarily be done via a writ of garnishment rather than via a setoff when the judgments are entered, because the judgments will not ordinarily be entered in both cases at exactly the same time.
The outcome of the A v. B case will only impact the outcome of the B v. A case when there is factual overlap between them in common law jurisdictions. Whichever case results in a final judgment on the merits first will bar all claims arising from the same general nucleus of facts that are present in another pending case. This is called "res judicata". Also, the specific conclusions of fact and law reached in the first to be decided case will be controlling as to the conclusions of fact and law necessarily decided to reach the outcome in the first case, if those issues are also presented in the second case.
But, since the key issue is when a final judgment is entered, not infrequently a simple lawsuit that is filed later will produce a final judgment before a more complicated lawsuit that is filed first.
I mean, can these scenarios merge in a single case?
Most countries have a procedure for consolidating cases involving the same parties under some circumstances. In Colorado, where I practice, there is a simpler procedure if all the cases are pending in the same court, and another more involved procedure involving a panel of the state supreme court, when the cases are pending in different courts. The federal courts in the U.S. also have such a process.
But, usually cases with the same parties will be consolidated only if they involve a common set of facts and circumstances.
In Belgium and Italy
Belgium and Italy should be almost identical on these issues respect, but I don't have a deep familiarity with either of them.
Both have civil codes derived from the Napoleon's Civil Code as opposed to the moderately different civil codes of Germany and Spain (almost all of the civil codes in the world has the civil codes of one of those three countries as a starting point). Italy differs more from France than Belgium, but it is unlikely that there would be a material difference on this point.
In the criminal law, any country with jurisdiction over a crime can prosecute it.
In common law countries (e.g. the U.S., England and Wales, Scotland, Norther Ireland, Ireland, Canada, Australia, New Zealand, to a lesser extent India, Pakistan and Bangladesh, and various U.K. territories), the usual rule is that all prosecutions by a single government (e.g. the federal government, or a particular state government) against the same defendant involving a single criminal episode in which multiple crimes were committed must be prosecuted in a single case with a single trial for that defendant.
But, if more than one criminal episode is involved, each may be brought as a separate criminal case with a separate trial in the respective courts with jurisdiction over the places where the crimes happened. So, if there is a car jacking in Denver County, a pimping operation in Mesa County, and a murder in Garfield County, all involving facts independent of each other, then there can be three separate cases against the defendant in three different counties.
If all of the crimes against a single defendant were committed in places that are in the jurisdiction of the same court, the prosecution could decide to try all of the outstanding cases in a single case with a single trial, but the defendant could usually object to this and have the case split up if presenting multiple unrelated charged would prejudice the decision making process on each individual count (e.g. by making the defendant look like a serial offender despite having no prior convictions of any kind).
The details above may not be precisely the same in Belgium and Italy, but they would be similar.
In criminal cases, A's wrongs to B never offset B's wrongs to A (except in the case of affirmative defenses such as self-defense involving the same incident). In terms of legal theory, this is because a criminal cases is not brought on behalf of a particular victim of the crime. Instead, it is brought on behalf of "The People" because that person violated a state or federal criminal law. If A criminally victimizes B, in the criminal law context, A harmed the People once. If B criminally victims A, in the criminal law context, B also harmed the People once. Two harms to the People don't cancel out and both can be prosecuted by the prosecuting attorney's office, usually in different court cases if the facts are unrelated.
This is legal theory would be mostly the same in common law and civil law countries. Some common law countries and some civil law countries allow victims of minor crimes rather than the state to prosecute these crimes in some circumstances, but that isn't terribly common in practice even in the minority of jurisdictions where it is permitted. And, even then, two wrongs don't make a right and both parties who committed criminal wrongs can be prosecuted independently of each other.
This said, it wouldn't be terribly uncommon in the U.S. for plea bargains to effectively cancel out criminal charges by dismissing charges against both parties when both parties committed crimes against each other, even though this is not recognized as a formal legal rule.