Training and leadership matter, but hiring is a big factor.
The lion's share of the problems flow from cops who unravel when someone fails to submit to their authority and commands, where the insecure cop is obsessed with "follow/comply with my instructions or else ..." regardless of the situation or risk threat.
[Not talking about justified felony car stops]
Good cops adapt and read the terrain during encounters. Just because a motorist stopped for speeding or an expired tag asks too many questions (Why are you stopping me? ... You had no right to stop me ... I don't feel comfortable ... why do I have to give you my name and address?), it does not follow that the use of force should be triggered or escalated.
Tone, empathy, respect, are important tools.
Not an easy job being a cop. Not lost on me.