A review of murder laws will clarify when a killing should be treated as manslaughter rather than murder, the Lord Chancellor has said.
The Lord Chancellor said he was against US-style murder categories
Lord Falconer's comments come after Andrew Wragg was found not guilty of the murder of his son and received a suspended sentence for manslaughter.
Jacob, 10, had an incurable disease and Mr Wragg said it was an act of mercy.
The review for England and Wales will assess when defences such as diminished responsibility are applicable.
"I think the public, and the jury and people involved in these cases need to have a clearer picture of where the line is to be drawn because the discussion we're having now reveals to some extent the difficulties of where that line is," Lord Falconer said.
The Home Office's homicide review announced in July is considering when the law needs to make clear when partial defences to murder, such as diminished responsibility or provocation in domestic violence cases, should be accepted by the courts.
"The law can never condone mercy killing, because the law is there in part to protect people who might be killed," Lord Falconer said.
"It's identifying that the pressures on people sometimes get so great that while they shouldn't have permission to kill, they do have a partial defence to murder which reduces it to manslaughter."
Lord Falconer said he was against introducing US-style "degrees" of murder but "a clearer set of circumstances as to where the dividing line is" was required.
On Monday, Andrew Wragg, a 38-year-old former soldier, of Worthing, West Sussex, was given a two-year prison sentence, suspended for two years, at Lewes Crown Court.
He had denied murdering his 10-year-old son Jacob, who had the degenerative disease Hunter Syndrome in July 2004, but admitted manslaughter on grounds of diminished responsibility.