It was illegal. The German criminal code has two sections for murder, § 211 Mord (the more severe one) and § 212 Totschlag (the less severe one).

In the original version from 1870, Mord was killing a human being with deliberation (“mit Überlegung”) and Totschlag was any other intentional killing of a human being.

In 1941, notably after it had been pointed out that what they were doing was Mord (Wikipedia), the National Socialists changed the law so that § 211 only applied under specific aggravating circumstances (e.g. cruelty or base motives).

That remains unchanged in the current version (English translation), except for the sentences.

Those murders are still prosecuted today. For example, in 2022 a former secretary at a concentration camp was sentenced to two years on probation for assistance to murder (Mord) in 10,505 cases (the court applied juvenile criminal law since she was only 18 and 19 years old at the time).