Bullying was part of it, but it goes deeper than that. I've researched Columbine on-and-off for 15 years, and my best guess is that they did it because they felt alienated from others, wanted revenge against society for not having what they thought they deserved (girlfriends, popularity, good looks, etc.), and individual predisposing factors (e.g., depression, suicidal thoughts, low self-esteem). They felt powerless and used violence as a means to assert themselves. There are also cultural issues involved as school shootings are, for the most part, a uniquely American phenomenon. For example, gun availability and rigid notions of masculinity should be implicated as well.
I don't think they "served justice" at all because they killed randomly and indiscriminately. Even if they had killed only their bullies, I still wouldn't view it as "justice". I don't think they would've fit in modern society either because some of the underlying issues are still in place and are even worse today.