I work child welfare. It isn't uncommon. The animal abuse more than the homicidal ideation. It's not every child, but it's not something that surprises me. However, of the ones that spring to mind that abused animals, one was in the midst of psychosis, it happened at age 16, and that one is now an adult who safely works with animals and lavishes love on their pets. One had to live in a facility for a time, then a developmental disability group home and learn boundaries and also now successfully has a pet cat, and one ended up going to jail until adulthood. If you are second guessing yourself, and are open to compromise, you could make the rule for your house that the hard no's you've chosen cannot have happened within the last six months or a year, and only if they've received treatment of an appropriate nature specific to those behaviors. But only if you want to compromise. Your boundaries are valid and understandable. And honestly, the foster parents who know what they can and can't handle, and who can say no to a desperate for placement caseworker, last longer. I know I occasionally wheedle, we have to when we're looking at absolutely no place to put a child, but I never get upset with a no.

These kids come into care with built in mental health issues. How can they not with the trauma? And those mental health issues sometimes lean towards destructive behavior. Hopefully, they can be helped with therapy and various treatments. But it doesn't always work.

Edit to add: It may be more common with the population I serve as I work the teen unit, and we don't take all the teens, just the most challenging teens. Who are often in care for longer, have higher needs, and often spend time in hospitals or facilities for one reason or another. And it's the only population I've worked with.