"cruel to animal" (animal abuse) is the issue here!

Every year, there are millions of cats/dogs euthanized in U.S. So the issue here is not killing the cats but how you did it. If you had taken the cats to an animal shelter, and if the animal shelter euthanizes the cats then there would be no issue here. If you had asked a lawyer (or a person with common sense) before going to the police, the situation would be very different. You did a bad thing and is too naive to put yourself in this position, there is not much anyone can do at this point.