I'd try not to, I'd try to reason with him or incapacitate him, but if he's a Marine then he's probably fluent in close quarters combat, so I may not have a choice. Either way, I'd probably be ethically cleared. The most common attack levied at those who've killed to protect their own life is the Two Wrongs Fallacy: if you accept that killing is wrong then you must accept that you were wrong to kill, regardless of reason. However, the Two Wrongs Fallacy has something of a get out clause in which a wrong action committed specifically to negate another wrong action can be justified. Or at least that's what this source claims.
Whether or not I would feel guilt over it is a different matter. If I felt that I'd willingly been the initial cause in the causal chain that had led to the man's death then I'd feel guilty.
Whether or not I would feel guilt over it is a different matter. If I felt that I'd willingly been the initial cause in the causal chain that had led to the man's death then I'd feel guilty.