I've written a number of posts about incidents where in my opinion police use of tasers was unjustified and unnecessary. But here's one where the police officer appears to have been justified, and the negative consequences were the fault of the suspect. In Australia a man ran toward a policeman carrying a lighter and a bottle of gasoline. When he refused to stop, he was tasered. Apparently since he had been sniffing gas and was covered with fumes, the taser caused him to burst into flames. The cop helped put the flames out despite being attacked by a bystander.
This incident is an example of where a taser is a reasonable option, despite the unfortunate consequences. It's unreasonable to expect a cop to disarm someone with a bottle of gasoline and a lighter, a potentially lethal combination. But at the same time there is no major threat until the person gets close enough. Rather than simply shooting him, a taser gives the cop an intermediate option between lethal force and putting himself at extreme risk. Unfortunately the rules for taser use tend to be very loose in most jurisdictions.