Tasers are a useful addition to the range of options available to police in subduing violent or resistant suspects.
The man in question had allegedly assaulted his his girlfirend, and had smashed some property. He was also known to be at risk of suicide, and at the time of his arrest was threatening no one but himself.
When capsicum spray proved ineffective, he was tasered at least three times. Tasering someone to death because he is threatening to hurt himself does not seem even a remotely intelligent or appropriate response.
If no one else was in danger, why not just back off until he calmed down?