Marcel Hesse, after three days on the run, was arrested late on Thursday after he went into a restaurant in the northern town of Herne and shouted “Call the police, I'm wanted”, Bild newspaper reported.
Authorities had been hunting for Hesse since Monday night after the video clip had appeared on the “darknet”, a hidden online arena notoriously used by criminals to trade weapons, drugs and child pornography.
Police discovered the child's corpse on Monday when they searched the cellar of the suspect, who has been described as an unemployed and socially withdrawn man not previously known to authorities.
After his arrest, police said Hesse led them to the remains of the second victim, a 22-year-old man, whom he knew from a job training college, and whose flat he set on fire after the murder.
After killing the child, Hesse had gone to the man's apartment, had a meal, played computer games with him and spent the night there, said the lead investigator into the killings, Klaus-Peter Lipphaus.
The victim confronted Hesse the next morning after seeing that he was being hunted by police.
“That was his death sentence,” said Lipphaus.
The 19-year-old allegedly turned on him, stabbing him 68 times.
Prosecutor Danyal Maibaum said Hesse “acted out of a lust to kill”, while Lipphaus described him as “extremely ice-cold, emotionless”.
Hesse also confessed to setting his friend's apartment on fire before leaving the scene, police said.
He told police he had “not give any thought” to whether the fire could put other people's lives in danger, added Lipphaus.