Ace's Motivations not JUST About Revenge
Although Ace definitely wants revenge against his shady partners, I also think that he just really wants to be a better person in the world and knows that's not possible if his horrible associates still have their hooks in him.
I loved the last shot of Ace looking wistfully at his horse, but I think that really symbolized his appreciation of these pure creatures, and also what he wants for himself. Mike is a very sinister figure (killing a man for no reason, not above using Ace's grandchild to trip him up into getting killed), and Ace just desperately wants to shake off these amoral leeches to run a more pure, giving business.
It's a beautiful character arc, and I'm sorry we won't get to see it finished.