Obviously God punishes not for vengeance, nor mere shortcoming. There are degrees in good and evil deeds. God's punishment is also a shade of His infinite Mercy, and some sort of a guarantee to stop miscarriage or non-fulfilment of the LAW and commandments' which may result in evolution and developement of MAN.
So also the doors of return of the SINNER should not be entirely and sternly closed. They should have access to some chances, and means to amend and correct themselves and return to salvation.
The intercession, in its most correct form is to look after such a balance, and be a means for urging the sinner to repent and return to God and piety; though some think that it may embolden the guilty to increase his making mischief. Some people take intercession as a sort of partnership to intercede for a wrong doer with a king or a tyrant ruler!