But they do, because Monica was symptoms are classic for battered/abused spouse syndrome. It isn't an excuse, it's the mental state she was in because what had gone down in her family. Also don't minimize that Victor had also managed to get her addicted to Three Eye in the process. It took a great deal of courage for her to even come to Harry in the first place ...
Yes, these
are all mitigating factors.
Yes, it took
tremendous courage for her to seek Harry's help.
They do not entirely relieve Monica of all moral culpability.
It's essentially the same as a drunk driver: once they're drunk, their judgement is impaired. But they
chose to be in that drunken state; and usually, knowing they'd be driving (home from the bar, etc).
I doubt Monica ever would have
chosen to be abused, or addicted. But there are moments in that downward slide where you can break free, where
she could have broken free. So long as it was just her, she is 100% the victim.
But her "victimhood" does not offer her complete moral cover when it puts others at risk. She made a choice; it may have been months before, or years. But she chose something destructive, and that choice led to her putting another person in harm's way.
... In the kitchen after the soul gaze Harry does ask her straight out why didn't she come clean with him in the office? Her only response was unspeakable fear, thinking she wouldn't be believed, for her own sanity she had no other words for him.. Harry didn't press her further on that point, if he felt she was culpable at all I think he would have pressed her or said as an aside afterwords that she bore responsibility, he didn't.
Harry was deep into his white-knight gallantry phase; Monica was clearly a victim. I doubt you could have gotten Harry to press more guilt onto her if you'd held a hot poker to the soles of his feet!