But it'd further our tech. Humanity has no hope of evolving past its war lust and appetite for destruction any time soon. Might as well destroy something on another planet that's trying to kill us than continue to wreck our own world and exterminate its species.
Fair cop, self defence is one thing... Like I say, after the proverbial bugs are gone, we'd most likely keep kicking chunks out each other anyway. And after a fight with another species, assuming we win, we'll most likely have developed even more effective ways to wipe each other out too.
but why would they not have war?
Expansion into space means colonisation, and colonies do not like to be under the control of a foreign power for very long. if they have colonized worlds it's more than likely those worlds would eventually rebel/try for independence which never ends peacefully.
Just because they're more advanced doesn't mean they wont still have war and fighting, it just means they'll do it on a larger scale.
I don't think they wont have 'war', they most likely wont be effecting it upon each other, is all. If they haven't gotten around to wiping us out, there must be a reason, and given that they have the technological prowess to get from wherever they come from to us I'd make the assumption they had the tech to wipe us out in seconds.