My gods, he would be utterly destroyed.
ESPECIALLY if it was in some way his fault. I don't doubt for a second that he'd be utterly suicidal.
I mean, even if he didn't fully go through with it, he'd be a shell. Everything he'd do would be half-hearted. If he even left his quarters, he'd be a mannequin. Ferrus would try to sit with him but wouldn't get anywhere.
At least, that's how I think he'd react pre heresy.
Fulgrim fans!
Theoretically, how would our boy react to accidentally/being tricked into killing his entire legion. Every single one of his sons. Dead. Because he was mislead.
Pre-Heresy Fulgrim, of course, but won't mind reading how would other versions of him react.