People do go out of their way to step on ants, because they are pests. If aliens see us as competition for resources or a potential threat in the distant future, it might be better to eliminate us now.

There is no such thing as "advanced ethics". Our human sense of morality evolved under our specific evolutionary conditions. Aliens would be completely different. Even worse, they could be taken over by Paperclip Maximizers (http://wiki.lesswrong.com/wiki/Paperclip_maximizer).

An interesting short story about aliens that are very different from us http://lesswrong.com/lw/y4/three_worlds_collide_08/