Kirsten,
I am sorry this happened. However, and I think Barbara, for one, touched on it a little. This ISN"T about cats, and Lily. This is about another adult refusing to respect you, your feelings, and your home.

When you take Lily out of the picture (and ship her to my house), and insert, say, a framed artwork on the wall, and a person that refuses to be careful with it, tosses it around, throws stuff on it...well, it is EASY to see the wrongness.

This person, be it your mom, your stepfather, or a friend, is acting very passive aggressive towards you, and trying to manipulate you and your feelings. This isn't about Lily. This is about your stepfather's own internal issues. If I were one to bet...I would say this is not his only 'quirk'. I would bet he is also like this in other areas...and it just hasn't registered with you.

To feel bad about losing your temper is one thing. And, if you feel so inclined, apologize for the way you reacted, but, not for your actions.