To put it briefly, Eric was a psychopath who was obsessed with violence and formulated the attack, which was to be a bombing, for years. He fantasized about numerous, high-scale attacks, strived to outdo any previous instance of American domestic terrorism with hundreds of fatalities, and kept a detailed journal of every little facet of their plan. It was his idea. It could be said that Eric almost singlehandedly carried out the attacks. He was sadistic, without empathy and saw everyone as inferior.
Dylan was a depressive tagalong who simply wanted a way out of life. He looked to Eric for approval for everything. He was good at heart and almost chickened out more than once. He wrote about love and crushes in his journal and simply saw himself as a failure. He put very little toward the attacks.
Of course, these two individuals are dead now, so we can't question them as to their real motives.
2. One of the most tragic of attacks the nation has ever seen. On 20 April 1999 Dylan Klebold and Eric Harris entered their high school in Littleton, Colorado and murdered 13 people - 12 students and a teacher, before taking their own lives. Blame whomever you want, but they were the ones to pull the triggers and set up the bombs. Sadly, few really learned the lesson outside of Littleton.
Moral is "Do unto others"