Perhaps you don’t quite understand what mortal sin is. It is a deliberate, informed choice to commit an action that is objectively grave matter. To do so means that the person is knowingly and willfully choosing to turn away from God. If he dies unrepentant of doing so, he will indeed go to hell forever – not because God has sent him there, but because he himself has chosen to separate himself eternally from God.
It is also possible that you may not have a fully adequate picture of humankind if you think that God owes anyone salvation because they are “nice.” Holiness is not niceness. Niceness can flow from holiness, but cannot substitute for it. Saints are men and women who know that they are sinners and seek to become holy through reliance on God’s grace, ordinarily given in the sacraments, and on doing the will of God.
