Okay, so it's great that you can see that you would feel so much better if he was actually dead. That would be because you would feel more free to let go of the bad memories and questions about what happened and why, right?
Losing a loved one is terrible, but we know that sometimes things happen for a reason that we simply can't explain. Sometimes young people get cancer and the doctors can't fix them. It hurts, it's hard, it's unfair but at some point we let go of the need to spend forever turning the question of why over and over in our minds or poring over that person's history for clues of where the cancer might have come from, because we know that sometimes there just aren't answers to be found, and even if we did find the answer, it won't bring the person back.
The same is actually true of what your ex did. He made choices that you just can't comprehend, choices you would never make. The honest truth is that no matter how long you turn these questions over and over in your head, you will probably never understand. How could you? It's just simply outside your range of conscience. There are probably reasons if you dig deep enough – just like for the cancer – maybe your ex had a terrible childhood and never learned how to love, maybe he was born with very limited conscience or empathy for others (that's a partial definition of a psychopath), maybe he is just a garden-variety selfish asshole who will go on fucking up his relationships all his life, but knowing for sure what the reason is won't change anything.
In the meantime, the only person your anger is now hurting is you. Not him. As you say, he is going along with his life, completely unaware of or hurt by your anger. Your anger only exists inside you, and you are the only person who can make a choice to let it go.
You could stand in one spot boiling with anger at him all day long, thinking the angriest and most vicious thoughts about him, feeling tense in your body, your face getting hot, stamping your feet, screaming, tearing your hair, punching a wall... and at the end of the day, the only person out of you and him who has a sore hand and feet messy hair and a red face and tense shoulders and a sore stomach will be... you.
So my point is this: you could do YOURSELF a massive favour and kill him in your head, as that is the only place where he is still part of your life. Stop trying to understand or direct mental anger at him and just kill him off.
Have you ever heard the expression "He/she is dead to me?" You have the power to make that true for yourself. Sit down, get a picture of him in your head, maybe carrying a suitcase and giving you and your kids the finger as he legs it off into the distance. Then mentally send your kids away and visualise killing him off. I would go with something like him being run down by a bus, or maybe falling into a sewage pond and drowning. If you really have a lot of anger to let go of, perhaps you could mow him down with an AK47. Then, stare at the body in your mind, say, "Well, so long, fucker" and mentally walk away.
"But that's not REALITY, Cheeky!" I hear you protesting. But are you completely certain that the scenario you're holding in your mind is absolutely, categorically true too? In your mind, he's living it up, happy, "free" of his responsibilities. But that is likely to be just as much of a fantasy as is the idea of him being eaten by a bear.
The reality is that people who abandon their children and partners universally have some very dark and uncomfortable shit going on inside them and I can assure you that they take their demons with them when they go. Just because you can't see the karma doesn't mean it hasn't already happened or won't happen. But don't punish yourself any longer by waiting around for the proof.