It's not clear why she'd done that. But there is something in the reports that made me think... It looks that for a long while in the past she was doing quite the opposite - trying to talk him out of his suicidal moods, urging to seek therapy... Then something's changed.
Close relationships with suicidal people are incredibly difficult. They eat at you every moment of every day. You never let yourself completely relax, even when things seem okay. Every word, every intonation can be a sign of an impending mood change and the black emotional abyss behind.
It may be that she has become burnt out. Unable to cope, unable to see any other way out of it.
What she did was still wrong and there can be no excuse. But her motivation might not have been the sadistic attention seeking or control freakery as is our first instinctive reaction to assume...