My wife cheated on me when we were first dating with the sociopathic pedophile that had groomed and molested her from age 13. He had brainwashed her into basically believing what was good was whatever brought him the most pleasure and then he used her guilt for dating me to try to get her to do very fucked up things. These included having a webcam on for him to watch the losing of our virginities.
This may seem like a long story already but it gets even longer then that. Few people are in this exact situation but I had gone through mental abuse as a child and understood how he could get her to do the things he did. She prosecuted him but it took years for her to slowly get over everything. For a while she shifted the "good is partners pleasure" mentality to me and that took a long time to get her out of. She would think that if we weren't having sex frequently enough I'd leave her so if I was sick all of a sudden it'd be the end of the world.
Having experienced it personally it's weird to see someone else go through all these things. Her situation was much worse then mine. This did expand my mind quite a bit when it came to judging people. I have trouble even hating the pedophile. I think he's broken enough to need the government to take him away from society, but I wouldn't say he's evil. Just so selfish (or ignorant) not to care about the effects of his actions.
The more and more I've lived the more I've realized how fucking complex everything in this world is. It's hard sometimes to imagine that the person sitting across from you at a red light is a full person with a family, friends and lovers. There are a lot of issues in this world that need to be addressed, but these realizations have shown me that the largest cause for strife is dehumanization. It's easy to not really believe someone else is as complex as you, to simplify their behavior to the extremes of good and bad.
My wife didn't want to hurt me, but she did (in a particularly horrific way). Through all that she did love me and still loves me. All too often we judge the person and only derivatively judge the action. On top of that we frequently don't take into account the surrounding influences to the action.
We've worked out well and there's been no more infidelity. She's still sorry to this day for what she did and as I understand it one of the things that gnaws at her is how she got so fucked up in that situation. Looking back she knows what she should have done, but it didn't seem clear while going through it. Something like that is torture, looking into yourself and seeing someone you don't recognize.
Tl;dr: Life is more complex than a single action.