How will training change anything? I guess with better training he could have shot the autistic man holding a toy. Gosh, you're right! That would have been so much better. More, better training is crap. We need different policies.
Police in some other countries do receive specific training on how to deal with the mentally ill, and in general on how to de-escalate and defuse tensions. Remember the account of the 4 Swedish tourists, who happened to be police officers, calming down a fight on a NY subway? Training would have to take local contexts into account, of course, but surely there must be room for training on how to de-escalate, and how to deal with a mentally ill or intoxicated subject.