Well can't cry over spilled milk, so don't beat yourself up about it now. It's obvious you'll never do anything like it again so that's the main thing. Cat's are amazingly adaptable animals and even pet cats are natural hunters. We've taken in a fair few strays ourselves and they have a habit of finding and adopting people who'll take care of them. I'm sure they are fine. If you'd taken them to the RSPCA then they might have been put down, so although this is less than an ideal thing to have done, it's not the worse case scenario by miles.