if baptism is performed using water, by pouring or immersion, and the trinitarian formula is pronounced, with the intent to effect what the church believes about baptism, then anyone may baptize, even a non-Christian. The one baptized (or his parents in the case of a child below the age of reason) must intend the effects of the sacrament and make a sincere, informed profession of faith. If these conditions are met the soul is cleansed completely of original sin, and of any actual sin commited before baptism, yes including those as horrendous as the ones that put Dahmer in prison. The person becomes a child of God, receives sanctifying grace–the state of seeing and knowing the Truth that is God and belonging to God-- the soul receives an indelible mark forever identifying it as a Christian. Assuming the person dies in this state of grace before committing any further sin, he would be accepted immediately into heaven, literally welcomed to God’s own bosom, without need for purgation.
If this causes you pain, I suggest you spend some time with the people in the gospels, stories such as the prodigal son, the Samaritan woman, the woman who anointed the feet of Jesus, the workers in the vineyard, the lost sheep, those delivered of demons, the good thief, Peter and sundry other sinners.