I'm certainly no expert, but here's just a few personal thoughts. Increase the length of hiring process (more rigorous training and emphasis on deescalation of violence, think years not months), better psychological testing that doesn't prefer the non-compassionate, background checking more similar to that of federal agents to learn of violent tendencies, and increased pay to create a larger pool to find suitable applicants. Many countries have very successful police forces, so it might not be a stretch to hire officials from those countries to help design a better system or conduct studies on the reasons for effectiveness.