If only, when an officer is suddenly confronted with a man brandishing a knife, that man also handed the cop a full psychological evaluation of himself for the cop to read and learn before working on de-escalation. 

Cops get to know this, in the moment: Man with knife. Man with knife ordered to drop it and does not comply. Non-compliant man brandishing a knife. Non-compliant = unpredictable, and could attack at any moment without warning. It takes about 2 seconds to sprint 10 meters and stab someone in the chest fatally.  

Tasers fail sometimes. Sometimes they don't penetrate through clothing or skin. If an armed man charges you, you've got about 2 seconds to put him down or your children will be fatherless (or motherless) just like that. There is no second chance. Whether a perp is "mentally ill" isn't relevant. They are compliant or they are not. They are brandishing a weapon or they are not. They are an immediate threat to life or they are not.