Han Solo shot first as well, you are going to tell me you would not have done the same?. Again if Cap wanted to talk he would have gone alone, and everybody knows this. I seriously cant even imagine how can you pass invading some ones home with an army as a "good gesture will". "No" is also a possible outcome of a negotiation, if he was not willing to take no for an answer then he did not go there to negotiate and the Xmen did right in defending them selves.
In fact if anything, if you have an argument with cap and he shows up at your door, this shows you should actually shot him, chances are he had a sniper aiming at your head from the start.