Once again, situations change. As it stands currently, you are legally required to pay him maintenance. You can’t argue with that. It doesn’t matter what he’s done in the past, he paid you maintenance when the children were with you (even when it was 50/50) and now the living arrangements have changed, it’s your turn to pay it to him.
Look, I get that you’re bitter about the situation, it can’t be easy, but it is what it is - you need to accept that and focus on building bridges with your children rather than getting one up on their dad.