“police recount a clinician’s saying Yang was violent before the shooting on May 2”
Yeah, they knew that surely.
They knew they were going into an environment where someone with diminished mental capacity had access to bladed weapons.
They likely knew he was actively wielding bladed weapons.
They had access to less than lethal methods of self defence, tasers, body armour, mace, tranquilizers, superior numbers, training in hand to hand combat, fucking nets etc.
They chose to use firearms in lieu of these and shot a mentally ill person to death unnecessarily.