A former soldier accused of murdering his terminally-ill son "sacrificed" his own life to end the child's suffering, Lewes Crown Court has heard.
Doctors had said Jacob would not live beyond his 20s
Defence counsel Michael Sayers QC said Andrew Wragg stood to lose his job but the act was a way to resolve problems.
The prosecution claims it was a selfish and drink-fuelled act because Mr Wragg, 38, could not cope with Jacob, 10.
Mr Wragg, of Worthing, Sussex, denies murder but admits manslaughter on grounds of diminished responsibility.
Jacob had the degenerative disease Hunter Syndrome.
Mercy killing claim
Mr Sayers claimed the defendant stood to lose his £80,000 job as a bodyguard in Iraq, but the killing last July offered a chance to resolve financial problems and an opportunity to bring up his other, healthy son.
He said: "Andrew Wragg cracked on that final afternoon."
Mr Sayers said the father was "transfixed with the compulsion" to end his son's final degradation.
He added: "Whatever happened, happened out of compassion."
He said it was "crystal clear" the defendant had an abnormality of mind.
Andrew Wragg told police his son's death had been a mercy killing
In his closing argument, prosecutor Philip Katz QC said: "Mercy killing is a highly emotive label and normally applied to a situation where the person who is killed is really at death's door and may have been for quite a long time.
"No one would suggest that, however poorly Jacob was, he was in that situation.
"He wasn't hospitalised, he wasn't receiving oxygen, he wasn't really in that palliative state... he was making his own decisions."
He said the test for jurors would be whether Mr Wragg had an abnormality of mind that would substantially impair responsibility for his actions.
The trial continues.