FB my honest opinion is that it sounds like a bloody awful thing to do, to your children and your ex. Unless there were extenuating circumstances of course. That's the problem with human relationships, they're always so much more complex and difficult than they look on paper.
I know someone who split up with her husband two years ago. She just walked out on him, no warning, no reason, left with the kids (not for another man). He was heartbroken. Begged her to come back for months. Couldn't live without her. Couldn't understand why she'd broken up their happy home and family. Etc. etc. Everyone thought she was mad at first, then just a total selfish bitch. She's lost all her friends because of her utter selfishness and her inability to explain her actions.
What they don't know, is that before they were married, when she was 24, she was pregnant (with his child) and he made it very clear that if she didn't have an abortion, she'd be on her own. So she had an abortion. And thought she'd got over it. But as the years went by and she watched their other two children grow up, she became angrier and angrier and more disgusted and horrified by what they'd done to their first child. And it got to a point where she couldn't bear to be in the same room as him. She went to counselling, to try and "get over it" but what came out of it, was that she needed to get away from him and stop living out the lie of a happy family when there was this deep grief between them that he denied.
So I guess what I'm saying is that nothing is ever totally black and white. There's nowt so queer as folk, and you never know what has happened between couples. But on paper, yes, I'd say your mother did something that looks bloody awful. We all make snap judgements about things we know nothing about. 