They realize they are not quite so tough when they actually get themselves into trouble. They get beat up, almost rape someone, and stumble across a dead man floating in the lake.
These college boys realize that this is NOT their lives.
After looking back on the story, it seems like when they stumble across the dead man in the lake (while the car is being destroyed), the boys realize that if they continue down this path, this is where they will end up.
They realize that they aren't really as tough as they have always thought.
I've come across this in real life, having friends that think they are ridiculously tough, thinking they could fight anyone. And then something happens and really snaps you into reality, making you realize that you just aren't as tough as you might have always thought.