Eva888: 

Sobering to think any one of us could be saddled with dementia at any time of life as it’s not just a disease of the old. Wise decision for the family and everyone who believed his actions were wrong on moral and common sense grounds. At least the facts will come out in court and some guidelines laid down for treating such incidents with in future.

I like mad engineers idea of offering a smile and a cup of tea :) that should be the first response in all cases.

Being charged, isn't the same as being found guilty. The issue is, people have already decided he is guilty, so if he is found not guilty or of a lesser charge, it will be seen as the police protecting their own.

As someone who has been exposed to people with dementia, older and otherwise, those people can be violent, scary, and can hurt you (even the seemingly frail ones). Caretakers injuries of those people are common and frequent. A great deal of care is required when dealing with them. Whilst the inbalance of physical power seems huge from the outside, the facts aren't fully known as to what steps were taken prior to tazing her to de-escalate. 

From the outside it looks like this was a completely OTT response by the police officer, but I'll reserve my judgement somewhat till all the facts are in play.