Collective punishment has been historically defined as a matter of official state action.
What individual Germans do when Muslim outrages have angered them beyond restraint is an individual response.
If that turns into a collective statistic which concludes that barbaric Muslim invaders are finally getting their own back, then mox nix.
You know the history, right?
"Collective punishment"--- intentionally and indiscriminately killing blameless noncombatants and aggressors alike --- whether by individual, gang, paramiliary, or state action, is murder.