Although I am extremely angry that Welby didn't act in 2013 I do feel very sorry for him. He may have made a dreadful mistake but that doesn't mean he's a dreadful person. I'm not a Christian but I do believe in forgiveness and I think Welby bitterly regrets his "lack of curiosity" and only he knows why he took that stance. I think Welby is capable of punishing himself for his mistakes, he really doesn't need any help. Welby has had an impossible job, he's been trying to hold together a diminished church in the Western world (but with an active evangelical wing) that wants to be progressive and a thriving, growing church in areas of the world that has very different aspirations, totally impossible.
fwiw I'm not condoning anything, he made a huge mistake and (eventually) has done the right thing, although I can't help but think he wanted to stay on to try to make things better. Sorry but I will always try to believe the best before the worst, it's very good for my mental health!