First of all, don't be terrified, I know it's hard bust try to relax...
I know this will sound hard to believe but those kind of thoughts are normal.
Let me tell you something. The people who act on these thoughts never feel guilty, anxious or remorseful due to them, they enjoy them, they get gratification (often sexual) from those type of thoughts. The very fact that you are troubled by those thoughts, and experience guilt is a very reliable indicator that they will never be acted upon.
You will never "turn into a sicko", and you will never do anything that 'normal you' wouldn't like to do.
"A study showed that those instructed to suppress intrusive thoughts experienced more distress after suppression, while patients instructed to accept the bad thoughts experienced decreased discomfort."
Don't suppress them, accept them, let them pass through your head as thoughts, try not to let them disturb you, you'll never act upon them so you've nothing to fear! :)
I can never even watch horror films without getting the images stuck in my head, often I dwell on them for days and days, and they trouble me, so I know how you feel. I stay away from horror films completely now, and I'm 31! :)
Good Luck mate, hope all goes well.
Edit: When you mentioned "mentally ill", were you talking of psychosis or just mental ilness in general?
Intrusive Thoughs only start to move towards Psychosis if you think a higher entity or 'God' is planting those images/thoughts inside your head or guiding you to commit those acts.
My thoughts are normal!? Thats such a relief, I was worried I was turning into a serial killer or something! Thank you so much it will really help me relax. I will try and let the thoughts just pass now I know they dont mean im insane!
Yes, there perfectly normal.
"... a questionnaire to healthy college students and found that virtually all said they had these thoughts from time to time, including thoughts of sexual violence, sexual punishment, "unnatural" sex acts, painful sexual practices, blasphemous or obscene images, thoughts of harming elderly people or someone close to them, violence against animals or towards children, and impulsive or abusive outbursts or utterances"
You wont turn into anything you don't want to, remember that and you'll be fine! :)