suppossedly the cop thought he was using his taser

Which appears to be the case. He was resisting being handcuffed, and they did what they would normaly do: one cop puts his knee into his back to hold him there and the other draws his taser and fires. It was not a taser however, and that's the issue. He was cuffed after he was shot, which is SOP for any dept no matter the circumstances.
The tasers were new to the dept (Dec), he was a 2 year cop, which is still rookie territory, and it's possible they had minimal training with it, and or, dealing with it on the belt, etc. Cop had had his first kid like 2 weeks before also, so his head was not where it needed to be which = ugly results.
The issue of whether cuffs, etc were all needed in the first place will have to be looked at in context to see if the entire event could have been avoided, but vids dont give context and we dont know what was said between the parties, etc.
Here in Boston, a girl was killed with a pepper gun used by a cop. Went right through her eye. It was really more due to lack of training than any thing else, and lack of training is a common problem as it costs $$$$$ and so it's often after such an even the depts orders all manner of training. I was at the Boston PD training after the pepper ball thing.