No, that's not the only reason. Generally speaking, one either has no warning that violence is coming (in which case one can't throw the first punch) or one does have warning (in which case it's possible to, e.g., walk away, negotiate, duck). On the other hand, none of us are perfect predictors of the future. There will be times when we believe the first punch is about to be thrown when it isn't. If we avoid aggression until attacked, it may be that nobody gets punched (or shot) at all. There's a reason that tit-for-tat is such a successful strategy in an iterated Prisoner's Dilemma - and that the only more successful strategies have been ones that punished defection less than that - and it's nothing to do with signalling.
I rejected a fully general moral prescription, not advice for what is often optimal decision making strategy:
Self-defence (or defence of one's family, country, world, whatever) is perfectly acceptable - initiation of violence never is. It's never right to throw the first punch, but can be right to throw the last.
It's probably easier to build an uncaring AI than a friendly one. So, if we assume that someone, somewhere is trying to build an AI without solving friendliness, that person will probably finish before someone who's trying to build a friendly AI.
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further edit:
Wow, this is getting a rather stronger reaction than I'd anticipated. Clarification: I'm not suggesting practical measures that should be implemented. Jeez. I'm deep in an armchair, thinking about a problem that (for the moment) looks very hypothetical.
For future reference, how should I have gone about asking this question without seeming like I want to mobilize the Turing Police?