He is best known as the creator and showrunner of the critically acclaimed but short-lived television series Lone Star (2010), Awake (2012) and Mind Games (2014).

In June, 2016, Killen revised Jeremy Slater's draft of Netflix's adaptation of Death Note.

While working at what he called "real jobs" (computer support, construction), Killen found it hard to give up screenplay-writing.

In a last-ditch effort, he returned to L.A., dressed up as a courier and dropped copies of a script at every agency he could find.

"That script probably wasn't deserving of getting produced," he told the trade publication Variety in a 2011 "10 TV Scribes to Watch" feature, "but it got me a teeny, tiny toe in the door.