The source you've linked to doesn't mention a stabbing, and nothing I've been able to dig up suggests there was a stabbing in the Köthen death. Death from heart condition was unrelated to the injuries sustained from a fall the victim suffered scuffling with the 18-year-old (which, yes, smells fishy, but I think you're casting aspersions on an unrelated coroner who was looking at a woman with a crushed ribcage because of possible malpractice on a local-to-you coroner's part, which is... Well, this is the sort of misinformation-spreading and stereotyping, and the harm it causes, that we're discussing in this thread).

Are you conflating the facts from two situations? Media coverage of the Köthen protests suggests there was a stabbing incident in Chemnitz close in time to Köthen.

All of that having been said: I think your point that Germany has laws discouraging you from casting aspersions on groups due to individual actions of the groups, and America has far fewer such restrictions, stands. Except... Germany's homicide rate is lower than the US. So Germany's clearly doing something right relative to its neighbor 'cross the pond (of course, it may not be this---but given that Germany had to deal with actual Nazis and the US has not yet had that challenge, I'm not in the habit of gainsaying Germany's strategy here regarding laissez-faire freedom of speech).