After watching it and speaking with several other LE instructors, absent other known issues, it would seem as if "mentally" the officer was drawing and deploying his Taser. I admit that I do not know where on his belt his Taser was or if he in fact had one at all, but the draw motion he made and hand positioning was very consistent with Taser training.
In Taser certification and recertification classes a great deal of discussion is spent on the placement of the device on the belt as to not create confusion in stressful incidents.
There had been three reported incidents of such confusion previously where suspects were shot with a gun when the intent was to use the Taser.
This was clearly an awful incident and the officer clearly owns the responsibility. His intent however, which will determine the level of homicide charge, will have to be determined in court.
Saying he was just an evil cop and wanted to murder this guy may feel good for some to say, but probably is not an accurate statement.