In a certain sense, yes. With the Reichtags fire decree of February 28, 1933 civil liberties and rights were suspended by the new regime. For the incarceration in a concentration camp, people were put into protective custody, a legal construct older than the Nazis and already extensively used in World War I and Prussia prior to that. In essence, it allowed Police to take people who represented a danger to society in custody in order to prevent them committing harm.

In 1937 this was amended by a law allowing police to place people in preventive detention, stating that if the police was convinced that someone was likely to represent a danger to society, they could be placed under indefinite arrest. This was mainly used against so-called "asocials" a heterogeneous group of victims comprised for example of beggars, alcoholics etc. etc.

Now, I am saying in a certain sense because the legality of it all depends on what kind of model of legal understanding you decide to embrace. At the time, a very en vogue understanding was a very positivist one, meaning that a lot of Jusrists embraced the view that whatever the state did was by definition legal because a state is unable to do something illegal. This has indeed changed, partly because of the Nazi example but in general, Concentration Camps were allowed under German law at the time. If you accept that law as legal is another question because it fails several tests we now have in place in democracies.

An important note on this is that murder was not legal even in the camps. In the very beginning in 1933 and 1934, there were even a couple of district attorneys who decided to investigate against camp personnel for murder. This however was quickly crushed by the Nazi leadership and no such actions were taken again later.

Sources:

• Nicholaus Wachsmann: KL. A history of the Concentration Camps, 2015.

• Wolfgang Benz/Barbara Distel: Der Ort des Terrors, Bd. 2 Frühe Lager.