Something somebody posted makes me wonder if the hit time was going to be killed even if they were successful.

There is nothing to clearly indicate they didn't know if they got Michael or not (60 rounds sprayed through a wall of glass doesn't give you the best view of things)

The shooters were unknowns - presumably recruited just for this hit, probably by somebody who wouldn't want to step forward for the credit even if they got Michael. - and clearly whoever killed them was ready to do it - I doubt slitting their throats was a knee-jerk reaction to their failure, even if it was assured. That sort of thing would take planning and pre-meditation