My heart goes out to you sweetie, it really does. It's hard to imagine why anyone would do such a thing to a living creature. Of course, we all know it happens every day, but that doesn't make it any easier to swallow.
Honestly, I would not be surprised if he did it to hurt you. People can be so petty and vengeful. It breaks my heart to think of such a reason to murder to innocent animals, but it could very well be true.
Take a deep breath, hold your head up high and congratulate yourself that this horrible person is no longer part of your life. If he would do something this cruel and calculated to his own animals, what might he be capable of doing to you? Yes it is hard to bear, and I know you feel you must bear the responsibility all alone. But know this for a fact my dear. This was not your fault, and you were not responsible for his actions. He and he alone made the choice to abandon his pets, knowing full well they would die a slow and horrible death. He is nothing short of a psychopath. And believe me when I say, you are much better off with him gone.
It will take time for you to heal hon, so give yourself the time you need. But don't waste another minute feeling sorry for this dirtbag. He doesn't deserve any concern from you.
I wish you all the best.
Gale