Why are some people so heartless in a relationship? What can make someone do this to another person?

I know a man who was with his "girlfriend" from the time they were about 16 or 17 years old. They grew up with one another and went through a ton of stuff together (she's White and tall/blonde/blue eyes & was dating a Black man in the 70s in a city that was 99% White). They had 3 kids, but never married and they lived in a great city and had a normal life, it seemed. Then, they all started getting sick somehow. The woman has Lupus, the man has lung and kidney problems, their oldest son has an illness and is very weak all the time and has to get surgery often, their youngest son is paranoid schizophrenic and their daughter is bipolar. I think it's all very sad, but maybe he was feeling resentment or too much stress, but he started cheating on her with a 19 year old girl who went to high school with his son! The affair continued for years until he confessed last year over the phone and told the woman that he's leaving her, and that the other girl is pregnant. To make it even worse, he proposed to the young girl, even though he never proposed to the first lady, despite them having 3 kids together. The 19 year old girl (who is now 27) has even been charged with domestic abuse because she's beaten him up! But he bailed her out of jail, and now they're expecting a second baby. The thing I don't understand is that he left his first girl to deal with 3 kids, and he says they're adults now so he feels like he doesn't need to have anything to do with them. He says they are all a part of his former life, and now he has a new life and new kids who need him. How can a man just walk away from a family with no remorse? She's sick with lupus, dealing with 3 children with illnesses on her own as a 50 year old woman, while he's married to a 27 year old girl now and wants nothing to do with any of them. I don't understand this though, how can someone be SO cruel to another person that they once claimed to love? My mom grew up with them, she watched them date in high school and raise their kids together and everything, now the guy just walks away.