Well, to make it less complicated (hopefully) I'll make it brief.
There was this guy who was friends with a family friend, last summer we spent a bit of time together etc, he wanted to be more than friends, I didn't. Eventually he realised this, one day had a psycho at me because I said no to him sleeping in my bed. He (though text) called me a ****ing b****, get ****ed said he would rather kill himself than spend a day with me etc.
In the few months we spent "together" he started doing stuff with my dad, not often though.
Anyway I showed my mum the texts, she confronted him which of course he apologised to her for, said he didn't mean it blahblah, promised her he would apologise to me for. I only found out a few weeks ago that she asked him to apologise, and she only found out then that he hadn't.

Well this was a few months ago and he still keeps coming over to our place, (not in the actual house) and recently I overheard a comment from him about having to babysit me or something... he said it to my dog but it was in front of my mum, also she didn't say anything to him.
My father agreed at the time of breaking everything off with him, that he would only ever go do sport things with him, which I said was OK, yet recently he has been here at least 2hrs a week and my dad has even begun inviting him here!

Basically I just hate it when he is here and my parents don't seem to care what effect this is having on me. I tried to explain, like how would it feel if you had someone you hated and couldn't stand even hearing their voice hanging around all the time. It just makes me angry even to think about him, let alone knowing he is 20 feet from where I live several times a week. I just feel so angry and sad and bitter and everything, not only because of him but because my dad doesn't seem to care how much it upsets me?

And a few months ago my mum made him a birthday cake. I nearly ripped my hair out. I once again said something like.. is it normal for mothers to make a cake for someone who has said horrible things to their daughter, never apologised, makes mean comments to, gives filthy looks to and likely still feels that way? It was like she was applauding everything he does to me and showing him it's OK to do whatever he wants..

I have had many arguments with my mother (which I hate) but she doesn't seem to get it. I said if there was someone who had treated her the way he treated me, I would want to go out and stab them, not make them a cake and I certainately would not invite them over if I knew it would affect her the way this is affecting me.

How old are both of you?

At any rate you obviously have a lot of anger toward this person and your family. You would "want to go out and stab them"? Not walk away, not break off the relationship, stab them?

This makes very little sense to me but maybe your parents don't want you to control what they do, who they do it with, who they invite into a house they pay for.

Maybe your father doesn't want you to pick his friends.

I think you need to speak to a professional to find out why you are so enraged. Nearly "ripped your hair out"?

Has your relationship with your parents been good up until now?

I am 20 he is 22.
No, not stab him, stab someone who is as nasty to my mum. Not literally, but yes, I would feel a lot of anger towards someone who causes my mum that much grief?
It has never been good with my dad, but with my mum, yes it's been fine.

I just hate feeling angry towards her for this.. Is my anger to her even valid? Because it makes me feel so guilty too and I hate hurting her!

Your parents can be friends and like people you don't like, it is their rights, you are a grown adult, how about moving out to your own home, then he does not have to see you.

I agree, I assumed you were about 12 or 13 reading this, it does not sound like a adult in the way you are acting toward someone.

Wanting or expecting your parents to behave a certain way, showing mom texts and so on.