I find it pretty darn pathetic that some people find it okay that this man died simply because he may have been unruly. If you cannot see that there are other options to deal with things besides the use of force that ended up in this case, killing the man, I feel sorry for you.
As many people here who have experience dealing with individuals with Downs Syndrome, there are certain ways to deal with them, and certain things that regardless of how good your intentions may be, simply are not effective.
I knew a number of people with Downs Syndrome. Occasionally, some would get upset and become very stubborn. Somehow, in all those years, I don't recall a single instance in which any of them ended up with a crushed larynx, seriously injured, or killed. In not a single instance, were the people dealing with those individuals seriously injured or killed. In nearly every instance, eventually the individual calmed down and cooperated. He was what, 26 years old? I bet this wasn't the first time he had an outburst. Somehow people managed. I'm fairly confident that this time, where he ended up dead, somebody didn't handle things appropriately.
If your job is to deal with members of the public, then it is your job to act appropriately given the information at hand. When the caretaker asks you to allow her a little time to resolve it, and tells you any physical attempt to remove him will make things worse, and use use physical force, unless you have more knowledge of dealing with these situations, that is failing the most basic principles of your job. When that results in someones death, you should be held responsible.
Also, as others have mentioned, if a DA wants an indictment, they get it. Heck, I don't know the laws in this particular state, but often a DA can bring a case to a grand jury multiple times until they get one. Based simply on the fact a man died while in custody, and his death was medically ruled a homicide, it would have been pretty darn easy to get an indictment. When a grand jury only sees one side (the side the prosecutor gives them), this is telling. Attorneys often like to play fast and loose with the facts.
Had the DA simply said "This man died in the custody of these individuals. His death was ruled a homicide by the medical examiner. It isn't your job to determine whether that homicide was justifiable or not. That is for a jury in the trial to determine, when they have evidence presented from both sides. I'm asking you to return an indictment", the DA would have got an indictment.