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.