I'm going to be perfectly honest - I get the sense that no-one on the patheon acted sensibly in the least. The more we hear from Solas and get from various Codex entries, I'm like 'why???'. It makes me MORE baffled that he's agonizing over the fact that HE was the one that ruined the elvhen empire. Kind of sounds like they did it to themselves.
Personal speculation, since we'll never know for sure. But my thought is that the guilt comes from the fact that he probably ignored the situation, or possibly even instigated conflict between the gods. He failed to speak up. Or if he did, it was too late. If the war was between the Forgotten Ones and the Elven Pantheon, then perhaps he put them into direct contact. Or if the source of the corruption was red lyrium, perhaps he originally goaded Andruil into pursuing monsters in the void.
That said- when you get to a certain level of guilt, sometimes you blame yourself for things that couldn't possibly be your fault. He can only account for the mistakes he made. We also know he has an issue with pride- part of Pride is imagining yourself to be that important.