AITA asshole for abruptly dumping my fiancée after she left my boys alone?

My fiancée and I have been together for a year. We're part of the same friend group. I have 2 boys, ages 8 and 10. She has a 19-year-old son.

My fiancée does not live with me but started staying over to get the kids used to having a stepmom.

Last Saturday, I was called into work. Fiancee offered to watch boys. I warned her that they are high energy, but she wasn’t concerned.

I didn’t hear from fiancée other than the kids were fine. I was too busy to constantly check in, but I figured if there was a problem, she'd let me know.

When I got home from work, fiancée and her son were gone. My boys were both home. The 10-year-old was in tears, which is not common for him. He said fiancée took her son out.

My boys’ side of the story is this: they wanted the 19-year-old to let them play with his Nintendo Switch Lite. He didn’t want to lend it out because he thought they were too young and hyper to be given a Switch. I have no problem with this because it's his Switch. They bugged him a few more times but let it go. they asked him to play catch, but there was a heat advisory out and he doesn't like heat. They started asking him about his Switch again. Eventually, fiancée took the phone they share away and put it out of reach. She told them, “If you can’t respect my son enough to leave him alone about his electronics, then you are obviously too young to handle your own electronics. Shortly after that, she took her son to lunch and out to go shopping and left the boys alone.

The 10-year-old was frustrated by the time I got home because there was a heat advisory out, and the house was very hot. He tried to adjust the thermostat, but it required a digital override that he couldn’t figure out. The smoke detector started chirping due to a low battery, which was giving both boys a headache and making the 8-year-old jumpy. Neither boy could reach the detector to change the battery, not even when standing on a chair.

My fiancée comes home and says the boys were driving her nuts so she left. Her excuse was that the 10-year-old could watch his brother. I told her she was wrong, that a 10-year-old is in no way mature enough to babysit, especially without access to a phone. She countered by saying in 3 years he will be a teen, and 13 years old is old enough to babysit. I said to her, “So should 13-year-olds drive because in 3 years they will be 16? Should 18-year-olds get drunk because in 3 years they will be 21?

I broke off engagement. Our friends are saying I’m an asshole and overreacting. I am starting to wonder if they are right. I am just so livid that she would leave 2 little kids alone without calling me. Everyone said she didn’t call because she was stressed out and needed a break.

So, AITA for dumping the woman over a bad afternoon?