The perpetrator was busy stabbing his victim so the shot he got off was difficult. Plus if the perpetrator is down and out and you shoot again (and are not a cop) then you get into trouble. Especially if you kill him. Of course a dead perpetrator would have been the best out come for the taxpayer.
Now at least one million will be blown trying and incarcerating him. Then all kinds of death penalty appeals for the next 20 years. Total expense will be about 3-4 million to the taxpayers
Also, no worries, the state of Oklahoma will finish the job on this moslem nice and proper.