Possessing what doesn't belong to you

The fascination with possessing something which doesn't belong to you continues to keep my brain occupied asking the why question.
This story out of Orlando reminds me of how many times while I lived in Lakeland, the swans of Lake Morton were stolen or even killed. Lake Morton has had swans for as long as I can remember and it seems someone was always vandalizing them.
What is it with taking or killing a creature which doesn't belong to you? I didn't understand it when I was a kid and I don't understand it now.