I read an interesting post on this. In effect, the meticulous planning, the purchases, all the preparations - those were the reason.  That actual killing was simply the proof of the skill involved in getting ready for the big night.

There has been no manifesto, no links to any group and the actual shooting had to be random at that range. He did not want to kill anyone in particular, he just wanted to kill a lot of people. 

At the end of all this, we are left with guesses and speculation but absent anything else, this makes some sense.