I'm to overreact and kill a kitten.
Why would you want to kill a kitten?
I always wanted to murder something.
Your impulse to kill a defenseless kitten is unnatural. Don't do it, and seek help for these unhealthy impulses.
What are you talking about?
What I mean is that your desire to murder something, even just an animal, may be strong, but it is not something that most people live with.  You can find a counselor or a therapist who will be willing to talk to you about why you feel this way, and they will help you to overcome the urge.  That way, you won't have to hurt any animal.