According to the Siegessäule website the suspect in the Nicky Miler murder has confessed to killing one other man and nearly killing a third. He also confessed to using GHB / GBL / Liquid Ecstasy to kill Miler. Miler accepted the drug from him and passed out. The suspect then strangled and robbed him.
The suspect also confessed to killing an acquaintance in his flat on April 6 and robbing him. He also met a stranger on Warschauer Strasse U-Bahn station and offered him alcohol laced with GHB. The man accepted and fell unconscious in a side street where he was robbed. A woman found the victim later and he recovered in hospital.
Full report on Siegessäule in German here.