Nobody here is saying police should open fire.
I would be actually in favor of this, using rubber pellets of course. And only in such extreme situations (because I'm aware that those kind of bullets can still be dangerous).
You need something that creates enough lasting pain and damage that the criminal in question does not just run away and loots the next shop in another street.
They are criminals, plain and simple. Outside of a demonstration, no one would bat an eye if the police would use any non-lethal force to stop a robbery, a weapon-wielding madman or someone throwing Molotov cocktails at the police.
From my point of view, the German police is too confrontational/aggressive with peaceful protesters and too hands-off with the black block. The latter is probably mostly influenced by their public image. Don't want images of police shredding a crowd of people I guess.