I doubt they would "step", because what "step" would mean here? Blow up Earth? But there are so much more readily available asteroids that it's not even worth messing up with planets.

On the other hand, ants are interesting, just for curiosity. We can certainly amuse and entertain, just like ants do, and we can afford leaving alone their nests, just because they don't really bother us.

But if we mean stepping on a single ant, not all the ants, then it's certainly possible aliens could kill a few humans inadvertently.