Could they have used something to disable him - possibly - but also remember he had indicated that multiple IEDs were planted around the city, so they had to proceed with the suspicion that he might have a remote detonation device of some type.
In that case, if he is not completely disabled he could possibly detonate multiple explosives. They had to take him out completely and quickly and this was the best solution to do that.
Hindsight is always 20/20
I think it was an overboard response as it cost them their bomb robot. What is wrong with using a good old fashioned rocket propelled grenade?