I've read so many theories on their friendship.
First, I hear that it was Eric who always ordered Dylan around, influenced him.
Then I hear that it was Dylan who ordered Eric around, influencing him, and convincing him to join on the killing spree he first came up with.
Judy Brown said that she realized how "Dylan was always trying to be there for Eric, and take care of him. Because Eric didn't have as many friends as Dylan..."
Then I hear that Dylan probably never really cared about Eric (unlike he did, which was reciprocated) and just used him to get the chance to kill himself.
Same thing goes for the bullying: Either the two were bullied a lot, or they were the bullies themselves.
Just makes me think, 'ARRGH! I DON'T KNOW WHAT TO BELIEVE ANYMORE!
'