Because he thought he could get an advantage out of it. It worked for a few days but the judge shut it down at trial.
The jury believing Sheila committed the massacre while everyone slept for no reason was always going to be very unlikely.
When Bamber committed the crimes and spoke to the police is when it mattered in this instance
What was going through his head and his reasonings.
Not a trial a year down the line, the night in question and immediate aftermath.
It also worked longer than a day to be fair, he wasnt arrested till a month later after Julie got dumped for an upgrade.