Assume a member of the government murdered another individual over some unrelated matter. The attacker is an important member of the US government, so the President classifies this information. Now, if one person tries to reveal this information in order to get justice, would the person who committed the crime get charged or would the person using classified material get charged with a crime like Snowden or both?
There are a number of reasons why the issue you identify wouldn't come up under the hypothetical facts presented, the first and foremost of which is that the President does not have the power to classify information related to a murder simply because it involves an important member of the U.S. government.
There is such a thing as a "state secrets privilege", however, which can be used to dismiss litigation which cannot reasonably proceed without disclosure of state secrets. This privilege was first officially recognized by the Supreme Court of the United States in the case United States v. Reynolds, 345 U.S. 1 (1953), and the fact pattern of that case is typical of the kind of circumstances where it might be asserted. Wikipedia recounts the facts in that case:
A military airplane, a B-29 Superfortress bomber, crashed. The widows of three civilian crew members sought accident reports on the crash but were told that to release such details would threaten national security by revealing the bomber's top-secret mission. The court held that only the government can claim or waive the privilege, but that it “is not to be lightly invoked” and that there “must be a formal claim of privilege, lodged by the head of the department which has control over the matter, after actual personal consideration by that officer.” The court stressed that the decision to withhold evidence is to be made by the presiding judge and not the executive.
In 2000, the accident reports were declassified and released, and it was found that the assertion that they contained secret information was fraudulent. The reports did, however, contain information about the poor condition of the aircraft itself, which would have been very compromising to the Air Force's case. Many commentators have alleged government misuse of secrecy in this landmark case.
The Reynolds case involved a deadly accident occurring on an allegedly secret national security mission, rather than a crime (e.g. an alleged murder over claimed infidelity between soldiers on a secret mission, or a "fragging" incident in which subordinate soldiers kill their commanding officer because they disapprove of the way he has led them). But, the states secrets privilege could also probably be used to shut down a state criminal case in which national security secrets would have to be available to one or the other party for there to be a fair trial in a similar manner.
In the case of a hypothetical federal criminal case, it would not be necessary to invoke the state secrets privilege, because the U.S. federal government could simply decline to prosecute such a case, or could dismiss a case that it had initiated by exercising prosecutorial discretion.
Now, if one person tries to reveal this information in order to get justice, would the person who committed the crime get charged or would the person using classified material get charged with a crime like Snowden or both?
It depends upon who wants to reveal the information and how it is obtained.
As a strong general rule, only government officials and contractors with security clearances are legally obligated to not disclose classified material. A person without a security clearance generally has no legal obligation not to disclose classified material even if it was obtained from someone with a security clearance who violated the law by leaking the material in the United States.
Likewise, if the information in classified material was generated independently of government officials charged with protecting its secrecy, for example, with statements from third-party witnesses who were not legally bound to protect national security secrets, this too would be lawful.
There are isolated instances when someone who has not voluntarily agreed to protect classified materials in order to get a security clearance can be obligated to maintain the secrecy of these materials. But, generally speaking, this obligation is limited, for example, to a legal duty not to disclose that a person has received a "national security letter" (i.e. basically a secret subpoena) from the government under the Patriot Act.
Still, even then, the duty is limited to not disclosing affirmative government acts directed at the person involuntarily prohibited from disclosing them, rather than a general duty not to disclosed information that is present in classified materials, or to not disclose classified materials generally.
The other main circumstance when there is a duty on the part of someone without a security clearance not to disclose information that is classified involves restrictions on the dissemination of technologies with military applications, especially cryptography related technologies and techniques. But, this would apply to the circumstances of an otherwise relatively ordinary murder only in the most contrived fact patterns. Usually, disclosing the existence of or the detailed workings of a classified technology is not necessary to resolve a murder case.
The U.S., in all of the respected above where disclosure of classified material or information contained in classified material is lawful is something of an outlier. It holds this outlier position largely because of its very strong protections of the freedom of speech, the freedom of the press and the freedom to petition government officials which is protected by the First Amendment to the United States Constitution (and made applicable to state and local government officials via the Fourteenth Amendment to the United States Constitution).
On the other hand, there are scenarios in which someone revealing classified materials would be guilty of a crime.
One example is that it would be a crime for someone with a security clearance to reveal classified materials that they are charged with protecting. The relevant law even prohibits disclosing or confirming facts in classified materials that have already been made widely known to the public through the media or otherwise, that have not been officially declassified. This was the crime Edward Snowden was prosecuted for violating, since he was a government contractor and former government employee with a security clearance who leaked classified materials.
It would also be a crime to pay someone with a security clearance to violate their legal duty not to disclose classified information (which would be a type of bribery) or to threaten someone with a security clearance to cause them to violate their legal duty not to disclose classified material (which would a type of extortion).
Likewise, is someone burglarized a government facility and stole or copied classified material in the process, or made an unauthorized hack into a government or government contractor computer system to obtain the classified material, that would also be a crime. It would also be a crime to solicit the services of someone, for pay or otherwise, to obtain classified information in this manner.
Both of these things, if done on behalf of, or for the benefit of, a hostile foreign government, would constitute the crime of espionage as well.
It is also not a crime for someone who suspects but cannot prove that there was foul play, to say so in the media and to petition government officials including legislative and executive branch officials, to seek justice for the victim of the suspected foul play.