Considering the journals, home videos, blogs, etc. the shooters had an idea of the "legacy" they were leaving behind. During the massacre itself it went far longer than I believe they expected. The media was obviously present throughout much of the ordeal. There are even cameras throughout the school.
They seem to certainly have some narcissistic attitudes, and what a chance to put that on display.
Were they just in another mindset, where all that was going on was NBK?