Im talking from a legal standpoint. It has to be right away. No time to plan at all even the slighest bit. If he runs away before you can kill him and you chase his ass down and cap him it can still be considerd first degree murder. If you go out on the town the next day to kill him it WOULD be first degree murder because you planned to do it.
If you find your wife raped and him standing over her and you react by killing him its still considerd second degree murder because hes not a threat to you. Whether or not you would be charged is up to the prosecuter.
Whether or not you get convicted is up to the jury and your lawyer may end up being able to play their sympathys and get you off.