I'm 14 years old. And I am having these thoughts.
About being a murderer well.. not so much like that I can't explain it.
I don't think I could do it at all though and I don't know what I want to be when I'm older and its definatlty not going to be a murderer.
Is it because I'm stressed? I'm not getting enough sleep.could it be the environment I live in. or the films I watch? They are just like scary horrors. Although I prefer comedys.
Why am I having these thoughts though. They are horrible. I don't want to tell my mum because she would disown me. I need to go the doctors and get tablets or something.
Have I got depression

I only want the awnsers from proper professionals not stupid answers like your not right and you sicko and stuff.
I just want to be normal again and get on with my life in school and stuff. I need to forget all this... Please hep me.. I'm desperate

I don't think a parent would "disown" a 14 y.o. for asking for help so I think you need to ask your parent(s) about seeing a psychiatrist(since you are under age)
Sounds like you need to discuss your problems with a professional and perhaps you do need medication to relax you more.
The one thing you mentioned--watching horror movies, stands out to me. Those kind of fantasies NO ONE NEEDS. They stay in your mind & one can miss the good, peaceful stuff that they can be enjoying. I use to watch them but now I watch calmer shows.
I do however, think that if you feel you are depressed, to tell your parents so, and seek professional guidance.

You really should talk yo your mum about the thoughts you are having she could help you through this!
First of all you have to stop watching horror films, have more social time go out with friends and take your mind off these depresing thoughts if this doesn't help then definitely go and seek professional help!

Maybe you should just tell your mother that you're depressed and need to see a psychiatrist and then tell him/her everything. You might have to talk to your family doctor to get a referral though. You sound like you're depressed - I was when I was 14. The fact that you fantasize about hurting/killing people but know or think you couldn't means you won't, but you're probably angry with someone like your mother or father, if they're divorced or separated you have good reason to be angry. I loved to imagine I was a navy seal or gunship/slick pilot in vietnam. I still read books written by soldiers in vietnam and the gulf war etc. but I only ended up throwing a rolled up tube sock and a roll of paper towels at my mom. I'm not making light of the situation, I just think you should work toward seeing a psychiatrist if you can. Even telling your family doctor how you feel is better than nothing. If you need to work up to telling your doctor please try calling kid's help phone - in canada 1-800-668-6868. It's 100% confidential no matter what you tell them.
In the US there are other possibilities like -
National Youth Crisis Hotline 1-800-442-HOPE
Nationl Adolescent Suicide Hotline 1-800-621-4000
Crisis Intervention Centre 1-800-333-4444
Help Now Hotline 1-800-435-7609
I don't know if these are confidential but if you call from a pay phone they would be.