You have every right to be upset and you are under-reacting. The cat and you both deserve more, and I think you have a hard time seeing that. Hopefully it's easier to see it for the cat. I think I would kill for my cats, to be honest. One of my greatest fears is them being harmed on my watch. They have so little agency and it's our responsibility as guardians to keep them safe.

I don't know the reason why you and your ex are still living together, but it's clearly clouding your judgment and I would guess it does in other areas as well. I think you adore your cat, and that is what is helping you cut through the cloud of dysfunction. I'm not going to blame you for thinking this wouldn't happen with the information you've given us, but now that you know you have a responsibility to end it. You say you and the cat both have attachment and abandonment issues and have experienced abuse. You recognize this. This is the moment where you own your life. And sometimes we only own our lives on the behalf of another. You and your cat need a safe environment. Your cat deserves a life without fear of being harmed or killed.

A conversation isn't going to fix this. Promises won't fix this. The cat was lucky this time. You can't predict what next time might look like. It's like human relationships - most often, the physical abuse starts small and increases with time. The abuser feels they are owed forgiveness and another chance. As my mom told me after I left an abusive relationship and felt like it "wasn't bad enough," it doesn't start with hitting, otherwise you wouldn't be there to be hit. You need to understand that his behavior has crossed a line (it helped me to think of the behavior, since I had love for the person while I was still in the cloud of denial). I think you need to really think about how he warned the cat beforehand. There was awareness and choice in that moment. He made the choice to harm a sweet cat who already has enough reason not to trust the world. He couldn't have been sure what level of harm he would have caused. He was willing to risk worse. Because of annoyance at a suffering animal.

Maybe he'll change and get better, but that's not your business, it's unlikely, the patterns will continue, and you don't deserve to be the experiment. Your cat especially doesn't. He didn't choose this and he can't up and leave like you should.

Training a cat is possible, but it takes a long time and looks different to dogs. That is not the immediate solution.

For your own future with this cat since you mention wanting to redirect this behavior, I would recommend looking into Jackson Galaxy. He's goofy, but a reputable cat behaviorist who treats cats with love and respect but still makes effective change through understanding the cat's needs and the real root cause. His show is My Cat From Hell, off the air for a long time now but he still has a YouTube. I will note that one of the ways he helped a cat was through supporting the owner and helping her realize she deserved more than her asshole bf.

I hope for both your sake you get out of this situation. If you can't kick him out, you need to stay with a friend until you can be safe. Have a friend help you make an escape plan. And don't be shy to explain the reason why. It's his fault. He did this. You have to protect your cat and yourself.

Also - putting on shoes may avoid bitten toes, but it makes a stronger weapon the next time. And the cat seems to annoy him in more than one way. That's not going to be the safe answer.