If Eric had never moved to Denver...

...would he still have ended up going NBK somewhere else?

Would he have tried to blow up his high school in Plattsburgh?

Cullen tells us that he was irretrievable - that he would have "gone wrong" no matter where he lived.

What do you think?

I seriously doubt it.
I think NBK at its core started with one depressed, bitter, angry, bullied, alienated boy meeting and over time becoming closer and closer to another depressed, angry, bullied, alienated boy.
They weren't alone anymore. Finally, there was another person, and someone they both really trusted there to not only truly understand all the feelings all of the above provoked but to reinforce them.
There was no one between the two of them to try to encourage the other or try to talk one of the two around because they both felt much the same and same things the same. There was ultimate agreement between them.
Over time, they became truly two halves of the same whole. They were bound together in a way that their other friends like Dykeman wouldn't have understood.