In Alastair Reynolds’ Revelation Space series, the victors of a “dawn war” far in the galaxy’s past were machines and they decided to wipe out any sentient life in the galaxy for reasons that aren’t important here, but not life in general. But by the time we came around they had degraded to the point that they weren’t doing a good job anymore and a few civilizations were just starting to slip out into space. Then they get detected and destroyed.
So the combination of wanting to destroy civilizations, but not all life and breaking down over time would allow it.