His actions seemed just fine to me.
Think of it in terms of our time...
A serial killer carrying a deadly weapon is caught in the act of trying to murder yet another victim.
The police taze them to minimal effect.
They taze again, killer still not going down. The police implore the killer not to do it.
When the killer lunges for the victim, do the cops shoot to kill?
I hope not, because for the comparison to work the weapon has to be specialized to work on the intended victim only posing no threat to the cops, so there's no reason the cops couldn't position themselves between the killer and the victim. The victim is also not hurt and unrestrained and can move away from the killer who has no long distance weapon.
I think the scene was just badly staged, everyone acted as if Yuta had a bomb and threatened to blow the place up. If the stun setting doesn't work, tackle her, kick her in the face, break her legs if you have to but don't just vaporize her.