So, let's say you unfortunately run over someone with your car. That person dies, while it can't be clearly shown if it was actually your fault, but you did kill someone, therefore you should die.
Not comparable to this single case, but like I've said before, we can't operate on the basis of Hammurabi in modern society. The killer will most likely get 20-30 years in prison, which is the Finnish life sentence, and on good behaviour he could get out in 10-15 years. However, if he is seen as mentally ill, he could be sent to a prison hospital/asylum of sorts and have very little chance of ever seeing the light of day amongst the regular folk.