Now you don't have to agree with me, this is just my opinion.
While Harris and Klebold's motives are to this day still unclear, this is my theory to that based on my 4 years of research. I believe they did it as an act of hatred and vengeance on society. Yes they were bullied but Eric in his journal wrote things like "I hate the fucking world" and "KILL MANKIND!" he also wrote about how the earth "should be given back to the animals" Dylan thought of society as "zombies" and also said "You've given us shit for years! Your finally gonna pay for all the shit!" I think not only was he referencing the bullies, but also society. Think about it, after the arrest, Eric and Dylan were probably traumatized from it, it brought shame on both of their parents and families. Eric hated cops for this, he wanted payback with the police force for this. He talked about his hate for cops in his journal. I don't know what Dylan thought of the cops after the van incident, but I think after this, Dylan also wanted vengeance, he wanted vengeance on cops, but maybe after some thinking, also wanted vengeance on bullies and society too. But Dylan did also want to die, so he not only wanted to die, but he wanted revenge on the human race too. Eric and Dylan while still planning made up others plans once the school was blown up, they would plan to become one of the worlds most well known domestic terrorist's. All in the name of hate. They would plant more homemade bombs on roads, bridges, police stations, gas stations, cities, maybe other schools, possibly go on more shooting rampages, since I think Eric had a dream where he and Dylan shot up a mall. And then, maybe if their plans for more bombings failed, they would then hijack a plane and crash it into NYC, as a final act of avoiding justice and revenge on the human species. I truly do believe they planned on acting on these plans, but the bombs failing to go off most likely made them lose all hope and confidence, so they would have to attack society a different way(shooting). Eric probably loved KMFDM cause of the lyrics, I think KMFDM gave Eric fantasies, like imagining himself killing everyone in sight,and bombing shit up. KMFDM drove Eric's hate, and made himself feel like he was godlike. Now i also do believe the fact that they were bullies themselves, but i think they did that as a sense of empowerment. "Why did they kill random people then if they wanted vengeance on cops?" because clearly, they were both misanthropes (except for a few exceptions towards certain people). People still don't understand that if they were bullied, why did they shoot everyone in sight? Like I said, misanthropes. Now, in case if any of you dont know what a misanthrope is, heres the definition. Misanthropy: A dislike of human kind. "But there are plenty of misanthropes in the world and they dont go out and kill people!" Well, sorry but there have been a handful of misanthropes who have enacted on their hatred. Examples: Pekka Eric Auvinen, Matti Juhani Saari, John Filip Nordlund, Carl Panzram, Adam Lanza, Sebastian Bosse, Kimveer Gill, the Harpe brothers, Elliot Rodger, Dmitry Vinogradov