Hey, the religion thing is good, I completely missed that one.
I've got some more.
-they can't help it but are poisonous to us (think the first people who came to the american continent)
-they are allergic to us (same, but other way round)
-they wipe us out by accident (e.g. the tidal wave of their spacecraft kicks Earth around and sends us into another ice age, or burns us)
I'd say that if you have a drive that can generate a lot of energy (i.e. travel from star to star with energy left for a quick detour around Earth) you have a weapon. However you put it. Just accelerate a rock from the asteroid belt enough to put it on a collision orbit, or see above...
