It's hard for me to argue this is anything but killing.

http://www.truth-out.org/opinion/item/25744-police-killing-of-yet-another-black-man-nearby-ferguson-protests-warrants-probe

Execution is murder by the state, but that term implies some degree of legalese.

I think 'killing' is the only word that comes to mind at this point - watch the video and judge for yourself.