Well, a problem is not with Ygerna, but with Damien. He was born without any memory of the past, started from a blank slate. He was raised by good guys, and he had no inclination to destroy the world. If at some point Ygerna had asked him to go dark side, it would have been his choice, and even then he wouldn't have gone all evil, destroying everybody and everything. Damien is a pretty rational guy, and starting a rampage without strong motivation is totally irrational. Even in case he had chosen evil, I doubt the war would have been that bloody and destructive. Revenge is much more destructive desire, then anything else. By executing Ygerna Divine did not leave Damien a choice - the guy loved her, and as any normal guy wanted to avenge her death. So all gratuitous cruelty that came out of this war is on Divine, and not on Damien. I personally am not an incarnation of evil (at least I hope so), but I would react exactly as Damien in similar circumstances.
