it's so easy to create your own original halloween costume(aka no costume of a famous person)
here is something i've thought of just now, right off the top of my head:
1. take a set of clothes that you don't want to wear anymore, if you have no set of clothes that you don't want to wear anymore, buy a cheap new one from a market or something
2. tear it up a little bit
3. take raspberry juice or any red substance that will leave blood-like stains on clothes, and pour a little bit over your torn-up clothes
4. take a fake knife or anything, really, that could look like/serve as a weapon, and also pour a little bit of red substance over that
5. put some on your skin too. preferably hands and face.
6. maybe mess up your hair a bit so it looks like you're an actual maniac killer
7. done.