bit like me saying i thought he was going to hit me so i punched him back first 
If you had a genuine belief that he was going to hit you, then your action of striking them first would be considered reasonable force 
I agree that, (purely, on the facts given here) it would be very extreme to "pre-emptively" tazer someone, but again, I don't know the in's and out's, so it's not for me to say whether it was right or wrong...
it's a fair cop guv 
like i said i'm sure they had good reason considering the armed robbery...
just think it's very dodgy in principal, say they got the wrong guy? or worse he had a medical condition :-/
In an ideal world their dynamic risk assessment would have considered these points ..
On a personal level, I agree the risks of deploying a tazer are huge.
Purely from personal opinion - I imagine they had a strong suspicion he was armed, to use the force they did..
The average bobby (certainly at the moment) won't have access to such a device, so I guess for much of it we must trust that the ARV involved had their reasons... and made their decision based on a lot of training and experience...