A man accused of killing a teenager he thought was a witch was said to have been "very quiet and quite calm" within minutes of the boy's death.
Eric Bikubi, 28, denies murdering Kristy Bamu, 15, whose body was found in a bath in an east London flat on Christmas Day 2010.
At the Old Bailey, Kristy's sister Magalie Bamu, 28, also denies murder.
A plumber who visited the flat on the day of the murder said he had not paid much attention to a "splashing sound".
Chris Sussex was at the flat to investigate the source of a leak.
In a statement read to the court, he said: "He [Mr Bikubi] didn't have any television on or anything and seemed a bit weird... he seemed very quiet and quite calm.
"I could hear splashing coming from the bathroom, I just thought it was someone having a bath and didn't pay it too much attention."
Twenty minutes later, paramedics and police were called to the Newham flat and found that Kristy had drowned and had no pulse.
Jurors were told that earlier, neighbours had heard chanting coming from the flat and angry shouting.
In a statement, again read by prosecutor Oliver Glasgow, nextdoor neighbour Navaratnarajah Nadarajah said he had heard "the voice of someone who was angry... he was furious and swearing for 10 to 15 minutes."
His wife Thusyanthi Navaratnarajah described "angry screaming", and another neighbour Umar Anzaar said: "It sounded like a fish market, by that I mean there was a lot of shouting, people all shouting at the same time."
The court heard that paramedics were called to the flat at about 12:00 GMT on Christmas Day.
Reyad Budaly, from the ambulance service, said Kristy had had deep lacerations to his head, however there had only been a small amount of blood around the bathroom where he lay.
When the teenager was moved to a communal corridor outside the flat, medics saw that he had a black eye, cuts to his lower back and left arm and some of his teeth were missing.
Mr Bikubi has admitted manslaughter on the grounds of diminished responsibility, as well as two charges of causing actual bodily harm to Kristy's siblings.
Ms Bamu denies murder and two charges of causing actual bodily harm.
The trial continues.