"When he calmed down, they let him up and he grabbed a kitchen knife. When his roommates tried to contain him again, he slashed roommate Bennett Cosgrove on the forehead and chin, the lawsuit says. "

I'm sure Cosgrove thought he was a threat.

For all of those commenting, who know nothing about the laws concerning the Use of Deadly Force:

If you are reasonably in fear of imminent Serious Bodily Injury or Death, Deadly Force may be used.  Civilian or Law Enforcement, the law is the same.