

Not entire groups of people based on generalizations.
Depends on the group. If the basis for the group identity is being criticized, that is valid. If there is institutional problems, criticizing the group for being complicit is valid.
It is only invalid when the criticism is aimed at generalizations that apply to less than half of the group, or are based on inaccurate stereotypes.
No, that user is just a jackass. There are a fair number of jackass mods and admins across instances, but far fewer than I was expecting.