In one hand, a cop is supposed to be putting his/her life on the line to protect and serve citizens.
In the other hand, they are trained to take anyones life before theirs if they feel their life is threatened.
It is really tough to expect a human being to be able to assess the mental stability of someone without a medical evaluation, which is obviously not possible in a situation like this. In a heated exchange like this, a cop doesn't know if the personal is mentally ill or even what type of mental illness, they are hopped up on drugs, someone who hates cops and wants to fight, or someone thinking they are going to jail for life and is willing to lose their own by trying to get away.
Point is, a taser indeed should have been used, maybe even multiple times, and should have been drawn well before a gun was touched
I read a report that said a taser was used, and it didn't stop the person. If a taser was used and failed, and if the suspect actually got ahold of the police gun - not just touched it - then I cannot see prosecuting a cop for this. Once the gun is out of a holster and in the hands of the suspect, you better believe every cop is trained to shoot in this scenario and likely to shoot multiple times to subdue the threat.
I am a little surprised by the cops reaction in the video because nobody seems to pull away in fear, like I believe most police would have done if a suspect had gotten the gun away from an officer, and I believe would have backed away while shooting. Part of me feels like the cops were just tired of fighting with the guy and as soon as he "touched" their gun, they knew they had a right to shoot and did it for only that reason. In which case, they are some fucked up cops who reacted out of line.
Conclusion: Not enough details to reach an accurate conclusion, but that's the case with most news stories.