Has he given you an explanation why he did such a thing??

What is happening with your aunt? She's got to be worried about her cat. I'm going back and forth here as to whether it would be a good idea to tell her what your dad did. Maybe she'd be able to "scold" him and make him feel bad enough he'd go back to get it.

Dad may have issues, but I'd let him know that no matter if he was my father or not, that I would not let him abuse/neglect another animal and I would have no qualms about turning him in for it. That's just saying you're ok with what he did... it's enabling him, and I'd be he doesn't feel badly about what he did either. If it were my father who had done something like that (although he wouldn't cause he's worse than me when it comes to rescuing pets), I'd turn him in in a heartbeat. I have no tolerance for animal abuse or neglect, and I don't care who it is that did it. Granted you are also younger than I am, and would have to live there with him and deal with the repercussions, so I can understand you being hesitant doing something like that, but someone has to put their foot down with dad.

Is there ANYWAY you can go and try to find the cat?? Do you have a friend that drives? Can you get it through dad's thick skull that he needs to go back and take you there so you can find the cat, or at least try to find it?? Heck, ride a horse or a bike, or something there if you have to, but at least get there. Until you've exhausted all available options, there is still a chance for you to help this cat.