I guess

Yeah I feel sorry for them too. Like they are literally going to go no where in life

Either they hate themselves or they genuinely think they're better than others.

Sadness. That's where it comes from.

i used to bully people because i was bullied before that but im a nice person now

They are a nobody at first and then they want to make some friends. And so been funny and talking about others or attacking people make them think they are special and have power.

Small penis?

They hate their life and want to take it out on everyone else.

I was a bully and I am quite successful.
Is not something that I am proud of. I guess it is just a spur of the moment. There's not always I reason, I only did it because I knew I could, and over time I realize my wrongs, over which I am kinda sorry (although I have to admit that some things I did still make me laugh).

Jealousy
Boring life, trying to make new friends by acting like an impressive person who can do whatever he/she wants.
Bullied themselves one way or another (being beated or abused by your parents for example)

how does being bullied lead to becoming a bully?

By trying to impress people

Insecure, jealous, or afraid of anything that is not the social norm.

I think one of any of these things causes people to become bullies. That or they are just mean spirited.

People hurt them so they want to hurt others
it also help if your larger then the average person

Family,TV etc.

Like, I'm sorry someone hurt you, but that's no reason to make someone else feel pain