Couldn't they just have gotten an 8-year old kid that grew up in a semi-downtrodden suburban neighborhood to wrestle the knife out out of Grandpa's hand?

How about in addition to having anyone who is going to be using a taser (or pepper spray) experience it themselves, we also IQ test potential police/security personnel... It might make it a lot harder to fill the ranks, but in the end, it'd easily be worth it. Remember: higher=better

Is there a 'standard' test for emotional stability?