It was the only time he knew everyone was definitely in the house.
It was the only time everyone would be at WHF when he was not working.
It was the only time everyone would be in the house and asleep. So unlikely to hear Bamber. This happened right up until several shots had been fired.
It was the only time everyone would be in the house and asleep. So even if someone did wake mid massacre, the person would be slow, injured and unprotected when reacting to Bamber's unexpected onslaught. This happened with Neville.
His only other execution options were when everyone was at the house and awake. Such as the last supper. But impossible to commit the massacre. And there would be no alibi.
Bamber only had a short window of opportunity after loading and hiding the rifle. Neville would find it sooner or later. He did not return home until 10pm, so went out a few hours later.
The dogs would be asleep.
It was the only time Bamber could claim to be in bed.
It was the only time Bamber could claim to have received a call from Neville. As he spent all day working on the farm.
It was the only time he could claim Sheila had gone crazy, going crazy during the day when WHF was not full, was not an option.
It was the only time Bamber could cycle to WHF without being seen.
It was the only time their would be no farm workers around.
The aim of an execution is to carry it out without encountering resistance. The only time to do this is when the target/s are least able to react. In Bamber's case it was when everyone was asleep. Sheila could have gone crazy at any time. Bamber had no time options for his execution.