that the aliens in Signs are peaceful. in the film you never see an alien commit a malicious act, it is just assumed they are (is it actually harming the boy at the end or helping him fight his asthma with some weird space inhaler? and he doesn't fight back against Joaquin Phoenix when he has the bat) all we know about the aliens is being told to us by the media or rumours so it can't be confirmed. The alien that was locked in the pantry would have been scared being stuck on another planet which would explain the odd behaviour. and why would they invade earth when there weakness is water? could they have run out of supplies and earth is a last resort?