A lot of horrors are clever (some are totally dumb), so they can think in way of "gains" and "losses", "gains" are what you said : mental or physical, pain, horror, and so. "Losses" ? I guess the inability to cause pain, so an horror's death is a loss to itself. There's a lot of example of mind twister horrors that can provide a benefit in short term to some metahumans, in purpose to cause them endless pain on the long term. And some horrors (Yrsthgrathe, again, it's one of my favorite with Bone crow and Artificer) can plan things on very very long term.
Anyway, it does not mean horrors didn't get involved in Earth destruction : if there is life on other planets, they don't need Earth anymore to come, and destroying Earth might have occured a terrible pain among humanity, so, who knows ?