Part of this is a training problem. I'm not making any comment as to the legality of what happened here, specifically, it looks like this guy just went ape-mode and, but for a miracle, has rendered this individual a vegetable, if they survive. That disclaimer aside, let me remark briefly on the broad training problem in US policing.
When PDs say "we need more training" this is just another way to say "we demand more funding". So, even the word "training" itself is broken, it's basically meaningless in public-policy discourse. The actual problem with a lot of police-training is the hyper-focus on officer safety. Of course, your safety is a primary goal in any altercation. And yes, under the law, deadly-force is authorized (same as for civilians) in certain circumstances. But unless it is a deadly-force scenario (which is rare), then the safety of the other individual also matters under the law -- a police officer can't just run down people with their vehicle because "I felt safer" because that would be homicide.
OK, on to the core problem with what little "training" that police officers do receive -- because the emphasis is almost solely on "officer safety", the potential injuries that can be inflicted by different maneuvers are not properly emphasized. Concussions, bone fractures/breaks, spine dislocation, etc. etc. are highly predictable outcomes of various types of CQB maneuvers. Police training that does not properly emphasize these dangers (meaning, most of the course would be focused on how to avoid them) is effectively eliding them. The officers train on the training dummies, and when they finally get "the opportunity" to apply the maneuver in a real-world scenario (like the above), they just blindly apply the maneuver they were taught, without any regard to how the human body is completely unlike a training dummy (specifically, far less durable).
There are better alternatives. This is not a plug for the Gracies but they've put a lot of thought into this particular problem and have developed a police-training product designed to address it. If these officers had been trained in this system, instead of just practicing ankle picks on wrestling dummies in a wrestling gym, unsupervised, this scenario is less likely to have occurred. Note that I'm intentionally passing over any particular facts about this particular scenario without comment other than that it looks to me like a crime was committed.