That's gonna be difficult

We even tried to dissect ET!

Then we show them Doctor Who and tell them that we only do that to bad aliens. Good aliens are our friends. (Maybe then they won't wanna kill us)

Just explain the motivation that they used in the movie. Simple

If alien beings are smart enough to get here, they are more than smart enough to understand those movies are: a) only movies, b) are made because we, as a species, consider anything from the outside a threat, and c) a reflection of our small-mindedness.

But see, if we went to another planet and they had movies of them killing vaguely different humanoid aliens, it would just look to us like aliens fighting other aliens, we wouldn't think it was specifically directed at us.