I think it is mitigating in the sense that one might find their actions easier to forgive, but to me that doesn't mean their punishment should be shortened. Which leads me to another question: is someone whom you don't (can't) entirely trust (because, for instance, of the long term effects of childhood abuse on that particular person), is that person forgivable?
I feel that I forgive them on one level by recognising that abuse has damaged them so much that they have no proper sense of right and wrong (or at least, not a useful sense, one that stops them behaving in unacceptable ways), but I still think they should be prevented, by losing their freedom, from repeating the offence.
So, I'm back to where I was before: sorry for them in part, but not completely.