It is perfectly human to want justice for killers but mercy for oneself. I believe they deserve justice, but I hope, for God's sake, they could somehow come to His mercy.
Part of the problem is that humans separate in our thoughts forgiveness and repentance. God does not. They are two sides of one coin, so that when we pray for God to forgive we are implicitly praying for the sinner to repent. Whether or not sinners after death have the ability to repent is God's concern, not ours.
Also, our prayer for God to forgive is for God's good and is not focused on the sinner at all. Those who become monsters like the hijackers are a great loss to God, and it is the extension of His kingdom to even them we are praying for.
Some object to this prayer because they think it implies that the State should forgive. Not so. The State has no capacity to even perceive or evaluate repentance, so it has no right to forgive. The state either dispenses justice or neglects justice.
Justice is a natural symmetry; it is perceived by common grace, which resides in all men. Mercy and repentance are graces which flow from Christ; all men do not have them. So when the State begins to even think about theological virtues it becomes a devil.
So the Christian prays for God to forgive the hijackers, just like He has forgiven me. I deserved death; He gave me life.
The State should punish the wicked men.
A lot of Christians seemed to have forgotten this during the Carla Faye Tucker sentencing.