I had asked him MANY times in days leading up to it to be SURE he recalled they had this big game and party and he assured me he would have them there EARLY. I had offered to help or bring them myself if he wanted and he said he was all set.
He knew it was really important to them, so just to get back at you, he hurt them.
My active alcoholic father, who died of alcoholism when I was still a kid, did something similar. He'd encourage my sister to beat up on me to get back at my mother.
He was a narcissistic d***head, and sounds like your ex is too.
Sorry this happened.