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Hello, I'm sorry to hear about this fellow's behavior.
Tomcats kill kittens for a few reasons. One is that a queen is less likely to return to heat cycles while she is caring for kittens.Another is this is a way to kill off the competition for food. And kittens mewling sound much like a food source which would stimulate aggression. Another is that it is possible that the kittens aren't his and by killing them he is getting rid of another Tom's kittens (genetics).Toms simply aren't built to care for or care about kittens. They are all about their own needs.
It is unusual for a Tom to kill a queen but Toms will kill another cat simply because they don't like that cat or sense that they are weak or sick. It's possible that because she was so close to delivery he may have smelled some blood, as queens will often lose their mucous plug a few days before delivery, or that he perceived her as sick or weak and killed her because of that.
It is really impossible to know why he behaved the way he did but I would not trust him around your other cats again.