Perhaps, but why ignore the concerns of others that were warning him it was time to let go? He could still have kept him restrained without the chokehold with the help of the two other guys who were holding Neely's arms down.
This is my concern. Not his intervening, not even then initial hold, I fully appreciate why he stepped up, and that he used the tools available to him, he was appeared quite confident in his ability to properly execute a chokehold, however if he was confident in his ability as he had received training in this particular move, he should have known when to let go. He is also trained to be aware of his surroundings, and did appear quite calm and in control from the footage I've seen, although I appreciate that is my perspective and I can't attest to how he felt on the inside, in which case I would imagine he knew that any immediate threat was over and the majority of passengers had left the train, and also that bystanders were warning him to stop.
I don't believe that DP intended to kill JN, but I am not sure he thinks it's wrong that he did, though I also get the same impression from this thread, but I do think he needs to learn something from Jordan's death and if this trial teaches him that then it's a good thing.
For what it's worth, I don't think he will be convicted.