The "bombs" they planted were untested and did not function. They had no strategy for dealing with responding police. They advertised their intentions in advance and should have been caught several times.
Columbine should have been a complete disaster for the perps, unfortunately fate, and the hapless police response, dictated otherwise.
Shoulda, woulda, coulda. The fact is, the attack was a success.
When this gets wrapped up, I predict there will be a similar trail of shoulda, woulda, couldas.