why did eric and dylan do it

why did they do it was it for revenge

They hated that school and everyone in it. 

'This is for all the shit you've put us through the past four years'

The way the jocks got treated like royalty and could do as they pleased. How E and D got picked on. That footage of Eric in Columbine where the jocks are approaching him and Eric ducks his head down pretty much says it all to me. They had literal shit thrown at them as well as ketchup covered tampons, glass bottles from jocks driving past whilst they were at the smoker's pit. If they ever fought back THEY were the ones that got in trouble. Teachers and the principle knew/saw what was going on and couldn't give a shit. Plus Eric wasn't from Colorado which turned out to be a bit of a problem for him according to Brooks. 

'I hate you people for leaving me out of so many fun things, and no don't fucking say "that's your fault" because it isn't. You people had my number and I asked and all. But no no no don't let the weird looking Eric kid come along. Oh fuck no.' - from Eric's journal.

They wanted to destroy what they could of that place and if the bombs worked it would've been so much worse. It's a shame they couldn't realise high school means fuck all in the grand scheme of things in life. But when pushed enough...well look what happened. I was bullied through primary and high school and whenever I fought back I'd get in shit and they'd make it out that I was the problematic one. But I knew that soon I could leave and never see those assholes again. No one was listening to them kids. Eric even put he had homicidal thoughts on his psych questionnaire and nothing was done. Give him some pills. (Yup been there, I'm on both Zoloft and Seroquel and it's a bitch to try get off) To me Dylan was so depressed and far gone that his depression turned into anger and hatred. WRATH. I believe he was the one who thought of the whole NBK thing, but not with Eric. 

That's my takeaway of it anyway.

Motive was unclear. At least officially.

ok
i know what it like to be like e an d