Ken,
Ken said:
So: How do I stop being nice?
One of the first few ways is to take your time to answer people. Bring some tension to conversations with people you don't like. Also, there will be people who will not like you.
Bring your eyes slowly and slowly turn your head when they say something stupid or when you both know you guys don't like each other.
There comes a point in time in your life where you learn that even if you are a cool guy, there are just people who are just not gonna like you, and there's where you start amplifying it.
Start out by being "rude" to your "lesser" friends.

You need to recognize you being an asshole in all the wrong place to know that you have started, and then hear it too often to know you have scale too much.
As an example, in my life, and i am touting my own horn, BEEP BEEP.
Most people know that Zac is happy, and he is a happy guy, and he doesn't bother people. But they also know that Zac can switch it up and be a complete asshole, and fight with you and make your life miserable, like literally make every sentence, every act with him is frame battles, which is what we humans really are.
But that's a necessary skill. That' takes time to build. And you do relapse. 
Build this in your life first, and then you learn how to be an "ass" with girls. or tell me that i am reading u wrong and you are a legitimate asshole. HAHA! Just that with girls, it is not working.
Zac