Viola would probably leave Alec over it. I think it would slam a lot of things into perspective for her on how destructive and evil he actually is...Not that she doesn't know, of course, but because she never would have expected this particular outcome. Both the destruction of what Viola knows deep down is an innocent life and the terror of realizing she can't really predict him would drive her away from him.

Alec, meanwhile, would be broken. He is in any AU where Isabelle dies before he does. He would withdraw from social interaction and seek self harm in a variety of ways, and maybe eventually he'd be functional again, but he would never get anything close to over it. I think his public persona would completely shut down and the only thing he'd ever put energy that is his criminal dealings, but even that would be ill-fated with how destructive he'd become. Essentially, he'd spin out and it would probably be the end of him.