Officers used the gun after the man had poured gasoline over himself.
....
"We don't know what ignited the fire," police said.
Doubt it was the lighter found on the porch.
It had to be the static spark from the cat on the other side of the house.
If a TASER is used in the normal method of shooting probes with the attached wires into the skin of the target, then it wouldn't produce a spark. The current would flow down one wire, through the target, and back up the other wire. It wouldn't arc, so there would be no spark.
However, if one of the probes did not penetrate the skin, and was instead stuck in clothing the current might arc between the end of the probe and the skin. This is intentionally part of the design, but since that isn't what normally happens, the officers might not expect it.
A TASER also has probes on the end and can be used like a normal contact stun gun as well. If used like that it would likely arc between the end of the gun and the target's skin causing a spark.
If both probes did strike the target and pierce the skin, then my guess is that the lighter on the porch is the ignition source. If the guy was holding it with his finger on the flint, he could have easily created the spark through involuntary muscle contraction due to being TASED.
In any case, I place full blame on the idiot who doused himself with gasoline and then resisted arrest. His actions were a threat of serious injury to everyone around him. His death is the result of his own actions, and officers should not be required to risk being set on fire in order to apprehend him.
Even if they fired bean bag rounds from a shotgun, they would have risked the burning powder expelled from the shotgun igniting the gasoline.