It's incredibly simplistic to suggest that people should just leave their marriages first, but in truth society is even less tolerant of someone leaving a marriage purely because they're unhappy than they are of someone who leaves to be with someone else. Take a look at the relationships board, and posters who post about their husbands wanting out. The response is almost always that he must have an OW and if not then he's a coward for daring to leave his wife and children.
I was married to an emotionally abusive man. He was convinced I would leave him for someone else, he tried to gaslight me into thinking that he knew I was having an affair (I wasn't,) he hacked into all my social media accounts, my emails, put key loggers on my computer to track what I was doing, put a tracker on my phone and then confronted me when I came home if I wasn't exactly where I said I was, I.e. Didn't remember the name of a pub, prevented me from going back to work, moved us 200 miles away from friends and family thus ensuring I had no support network, and then spent all his spare time away from me while insisting I should stay home.
Then I started talking to someone online about a mutual hobby, and talk became more personal until eventually we met up and yes, had an affair. it is without doubt one of the most regrettable things I have ever done because me having had an affair means that everything he did previously was cancelled out, and the wrong was all on me. Didn't matter how much he abused me before that, I was a cheat, and as such he was an innocent victim.
In my case it wasn't that I fell deeply in love with OM and needed to leave my marriage to be with him, but meeting him made me realise that I was none of the awful things my DH had told me I was over the years. So it gave me the strength to leave anyway. We didn't end up together, and that too was the right thing. And me and XH are now on better terms than we ever were, because I no longer have to feel afraid of him. He has a new partner now and more children, something which wasn't possible when he was with me.
As to the poster who believes that her SIL was responsible for her brother's suicide, suicide is an awful thing to happen in any family, and looking for someone to blame is natural. But nobody is to blame for the choice someone makes to deliberately end their own life. If someone came on MN saying they had caused someone to commit suicide no-one would support that view. This of course doesn't cancel out the fact that the SIL had an affair, but the fact he committed suicide was his choice and his alone. nobody else is responsible for that. otherwise it could easily be used as a threat to keep someone in a relationship for instance, and who would support that if someone came on here saying "DH says if I leave him he will kill himself," people wouldn't be saying that she should stay with him. This is no different.