For example:
Can they be convicted with criminal attempt?
The fact that the chosen means could never have actually killed the target does not preclude an attempt conviction.
See United States v. Dynar, [1997] 2 S.C.R. 462:
The only relevant distinction for purposes of s. 24(1) of the Criminal Code is between imaginary crimes and attempts to do the factually impossible. The criminal law of Canada recognizes no middle category called “legal impossibility”. Because Mr. Dynar attempted to do the impossible but did not attempt to commit an imaginary crime, he can only have attempted to do the “factually impossible”. For this reason, Mr. Dynar’s proposal that s. 24(1) criminalizes only attempts to do the factually impossible does not help him.
An example of a "factual impossibility" cited by the court was "impossibility due to inadequate means... For example, A tries to kill B by shooting at him from too great a distance or by administering too small a dose of poison."
That this man’s design is premised on a mistaken understanding of the facts does not make it any less his design. A mistaken belief cannot be eliminated from the description of a person’s mental state simply because it is mistaken.
In your examples, the person who took steps to kill a person via inadequate or factually impossible means could be guilty of an attempt. Of course, this is subject to proof of the required mental state (intention to kill) beyond a reasonable doubt.
However, the actus reus of attempt in Canada is that the accused must have taken "some step towards the commission of the offence attempted going beyond mere acts of preparation." You haven't described anything about the person who hangs out with a militia that would constitute a step beyond a mere act of preparation.
In germany, attempt to commit a crime is normally a crime (§23 StGB). For instance, when a would-be murderer administers a substance that is not actually poisonous to the victim, that would still be a crime. However, when the attempt is not feasible (untauglicher Versuch), the court may reduce the punishment or exempt it completely.
This is distinct from a superstitious attempt (abergläubischer Versuch), where the would-be perpetrator tries to enlist the help of supernatural entities in the attempt. Criminal law does not believe in those supernatural entities.
So your first example, the curse, would not be punished. The second example, the militia, runs into other criminal laws regarding criminal or terrorist gangs.
