He had trained and fought as a member of Al Qaeda.
Killing unbelievers is their main objective.
They had succeeded in instilling in him the will to fight to the death against impossible odds with no fear of death.
I say that that is good evidence that killing westerners was indeed his main ambition in life.
You could make him a ward of court, but you could not lock him up.
Would you want him sleeping in your house Kevin?
Where would you put him?