I was an OW. I'll be 100% bluntly honest.
How do you justify it to yourself?
I didn't need to.
I knew him for a while. So By default I got to know her. They had been married since they left school.
He was incredibly unhappy. She was/is a horrific person who seemed to just want him to be unhappy no matter what he did. I didn't understand why he would stay with her, she was as ugly on the outside as she was inside. She would call him constantly and scream down the phone at him about things out of his control. All she wanted was material things that he got himself into debt for. She would regularly use his children to manipulate him.
We were friends, then best friends, we developed "feelings" that turned into innocent flirting. Innocent flirting turned into love. I told him I wouldn't be the OW and he left her. We took things slowly and dated and he was wonderful. It was so clear he had never so much as had a cup of tea made for him in the morning. It broke my heart. Then I saw some of the messages she would send him and other things and I'm so glad I was the catalyst that got him out of that. Even if I wasn't the one he went on to remarry, I'm glad he could be happy because he deserved it.
What is it that makes you do it?
You're assuming women who end up as OW have deliberately sought out to be an OW. Sometimes it doesn't end up like that. You also have to accept that the OW isn't the one that promised to be with you.
I certainly didn't think "omg, yes let's split them up" how powerful am I?! The fact that he was married was awful and unfair.
How can you live with yourself knowing what it does to the other person?
Easily. I'm not a particularly attractive or spectacular human being. I didn't bait him, we just fell together and were very happy for a long time.
And she's a toxic dump of a human. I hope no she never inflicts herself on another person again.
This probably doesn't help you, and you're probably hurting. I know I hurt when I was cheated on, but I never blamed the OW because it was such a waste of energy. If it hadn't have been her, it would have been someone else.