He was detached from reality and completely lacked the capacity to feel remorse after stabbing innocent stranger Richard Hall to death at a Shropshire beauty spot last year, a medical expert said.
Christensen, 22, suffered from severe psychosis and has previously believed he was a god, Stafford Crown Court heard today.
Christensen, of Corser Street, Stourbridge, has already admitted that he stabbed complete stranger Mr Hall on Brown Clee Hill on August 13 last year – he denies murder by reason of diminished responsibility and his trial began this week.
Mr Hall, from Perton, South Staffordshire, was found on the hill with dozens of stab wounds and Christensen turned up soaking wet on a stranger's doorstep soon after.
The case so far:
The trial heard today from Dr Dinesh Maganty, an experienced consultant forensic psychiatrist who assessed Christensen and his medical history after his arrest.
He said that the defendant did not use drugs or alcohol but had an "extensive" history of being prescribed medication to treat psychosis and depression.
He was also asked about Christensen's "curious" eating habits, which he asserted were actually "much worse than curious".
The defendant had a tendency to eat pet mince and raw sheep liver, and had claimed to have killed at least one sheep to eat its liver.
But he was passionate about opposing poaching and "loved animals", and was devoted to his dog before the rapid deterioration of his mental health, the psychiatrist agreed.
Christensen was previously known as a "sensitive, caring child" who had a "clear pattern" of severely deteriorating mental health and irrational behaviour in the months before the killing.
"He lost touch with reality, there is no doubt about that," said Dr Maganty.
"He was clearly suffering with a severe mental illness, a severe mental disorder."
At one point he "thought he was god and the whole world was against him", Dr Maganty told the court.
Defence representative Mr Anthony Metzer QC said that Christensen showed no remorse after killing Mr Hall, and Dr Maganty said that he completely lacked that capacity and could not recognise that it was irrational.
Christensen's inability to understand the thoughts or desires of other people manifested as a total lack of empathy, Dr Maganty agreed.
By the age of 14 he had already decided that he wanted to kill a human being, the doctor said under cross examination from prosecutor Mr Adrian Keeling QC.
The psychiatrist also said that today Christensen does "not even vaguely resemble" the man he assessed in November.
He is currently detained under the Mental Health Act in a medical institution because he requires urgent treatment, Dr Maganty told the court.
Christensen denies murder and the trial continues.