There's honestly so many possibilities of what could've happened if one backed out last minute. I could see that maybe Eric would've went on with it but the mixture of Eric and Dylan is what really made Columbine, Columbine. The shooting we know today. The perfect storm.
I agree with what
[You must be registered and logged in to see this link.] said. At the same time, if neither got the appropriate help needed, their lives would've ended as a recipe for disaster either way. Dylan might've committed suicide later in life and Eric might've done the same or would've gone on a rampage before suiciding.
Honestly, if they both decided to back out, I wonder what they would do with the propane bombs. To try and set it off only to realise it isn't going to go off. Eric may have well hid ammunitions but where are you going to hide a propane bomb?