A few commenters have wondered how Germany could go about excluding migrants who commit crimes.
I'm no expert on German or international law, but here's the 1951 UN Refugee Convention, Article 2: "Every refugee has duties to the country in which he finds himself, which require in particular that he conform to its laws and regulations as well as to measures taken for the maintenance of public order."
Beatings, assaults, and threats of murder are against the "laws and regulations" of Germany. Any country has the right to exclude foreign nationals who commit serious crimes on its territory.
I would say that Germany could pass a law tomorrow saying that any asylum seeker found liable of serious crimes on German territory has forfeited their right to asylum in Germany and may be excluded. Article 16(a)(4) states:
In the cases specified by paragraph (3) of this Article and in other cases that are plainly unfounded or considered to be plainly unfounded, the implementation of measures to terminate an applicant’s stay may be suspended by a court only if serious doubts exist as to their legality; the scope of review may be limited, and tardy objections may be disregarded. Details shall be determined by a law.
The law would specify that persons who are found to have committed violent crimes in migrant shelters can be deemed to have "unfounded" cases for asylum, since they have violated the 1951 Convention and demonstrated a tendency to violence. It's a semantic stretch, since there's no necessarily link between their behavior in a German migrant shelter and the reasons for their alleged persecution back home. But any legal system is full of such imperfect logical fits; there are dozens in German law already. And in any case, the law's purpose would be very popular and would make sense, which is something you shouldn't discount.
The key thing would be to make sure the migrant got a hearing that satisfies minimum standards of fairness. Since migrants aren't German citizens, have no legal residency status, and are not being threatened with prison, I don't think you would have to give them anything near a full-blown trial on the merits. You would give them a brief hearing, an appointed lawyer, and a chance to speak. The standard of proof could be something like clear and convincing evidence, not evidence beyond a reasonable doubt.
The result of the hearing would be a finding that the migrant has engaged in acts contrary to the 1951 Convention, forfeited his right to seek asylum in Germany, and therefore should be immediately deported.
Of course, then you have to actually deport them, which brings a whole host of complications of its own. Some countries won't be all that eager to take back violent religious fanatics.
This, my friends, Is. Why. You. Screen. Refugees. Before. Letting. Them. Into. Your. Country.