"My starting point is compassion," he said before taking part in the debate. "Put yourself at the bedside of someone suffering terribly, particularly when people could have, or would have, requested help to die and been denied. Prolonging agony doesn't seem to be right."
He conceded his views are at odds with the Kirk hierarchy but said his comments have attracted a lot of support.
"Among my congregation, not everyone agrees but many, many do," he said.
"I received cards, letters and emails from all over the world after speaking about this, which is encouraging."
Opinion appears to be split on whether assisted suicide should be legalised, according to responses to a consultation on Ms MacDonald's Bill, published in November. Interpretation of "substantive" responses by individuals and organisations suggests 59% are in favour and 35% are opposed, while 6% are neutral.