Groups wiped just because, would be the jews, the armenians and a long list actually (1). You could argue that some of them is because territorial fights, but is it not that also the reason of the "Three body problem" aliens?

About why alien civilizations could decide to engage in this kind of thing: if you are paranoiac enough you can frame it like a preventive attack, if you are reasoning that you will be fighting them for resources in the far future.

About how, there are many ways. The most obvious, though expensive, is just accelerate a lot of stuff (ideally antimatter), the closest to light speed as possible, in a interception vector with the undesirable solar system.

This is not the kind of thing I was planning to think about in Christmas eve.

(1) - https://en.wikipedia.org/wiki/List_of_genocides_by_death_tol...