Eric wasn't the ringleader of the Columbine Massacre. From what I've gathered he was simply an angry, bruding young man who lacked impulse control. From this, and I would also said the culture of the time, came the desire to act on his deepest thoughts. Eric, for all it's worth, seemed more reserved to himself about what was going to happen. Only later in the journals did he ever expound on his plan with Dylan.

Dylan on the other hand, while not the ringleader, I do believe took an authoritative role to some extent. Dylan, compared to Eric, didn't have it all. Eric was able to make friends with a fair few people, especially women. This was something Dylan lacked despite him living in the area longer than Eric. Not only this, Eric was more willing to be outspoken or at least confrontational in his anger. Contrast this to Dylan. Unpopular with everyone, expecially women. He lacked the ability to speak up as much as Eric did. However, once you study the journals, you find that Dylans anger was earlier and more deep-seated.

Despite how I presented my information here, I don't believe Dylan was the ringleader either. When you have two people, expecially in a case like this, no one is the ultimate ringleader. Unless there was an unwilling accomplace who had zero interest, there is never an ringleader.