"It is a totally horrific, disgusting event," the judge said when jailing him.
"It is horrendous to consider how terrified the woman must have been," he added.
The 19-year-old man stabbed the 82-year-old woman 40 times with a knife after being left alone for several days in the family's Bielefeld home for the first time by his mother and sister.
He was seemingly frustrated at having been rejected as a trainee paramedic and had stayed in and watched the violent horror film Saw, the court heard.
He then donned the mask, grabbed a knife and attacked the woman, who lived in the same building.
The judge said that although the man was demonstrably immature, and would thus be tried as a juvenile, he had committed murder and would have to serve a long prison sentence.
The man denied the attack, despite the physical evidence - his clothes were splattered with the woman's blood and tissue, which could only have come from pulling the knife out, the court heard. There was also blood on the mask which was found in his flat.
The man's defence team said there was no reason for him to do such a thing. He still has the right to appeal the sentence.