Why he shot them

The Complete and Total Loser is a failed journalist. He was the butt of jokes in the newsroom he worked in and although his leaving was by choice, he's certain most of his coworkers were glad to see him go. 
With that personal history, which was over a decade ago, the Loser understands the rage Vester Lee Flanagan II may have felt at his failure to succeed in what is likely the only job he ever enjoyed. But of course, nothing—ill treatment, unfair termination, outright humiliation—justifies murdering people. 

Your occupation is not everything. It pays your bills and for a very lucky few it allows a life of luxury and financial ease. But it is not who you are, it does not define you, it doesn't matter that much. It's just a job. The victims, Alison Parker and Adam Ward, will be remembered as good people first, competent workers second. Flanagan will be remembered only as an angry, mean, nut.