I believe that the confessional seal is inviolable because the priest who is forgiving people for their sins is acting as Jesus, so really we are telling Jesus our sins. Just like the Eucharist at Church when the priest says "This is My Body, which will be given up for you." He doesn't say, "Jesus said, This is My Body, which will be given up for you."
Remember that if the priest does not feel that the person is not genuinely sorry, he doesn't have to give absolution. So if a murderer or a child sex abuser asks for forgiveness and the priest doesn't feel that they are genuinely sorry and are just abusing the sacrament then the priest can tell the police if he hasn't given absolution. Also if the priest knows what a person is going to say then he can also refuse to give them absolution, but once it is given, he isn't allowed to break the confessional seal. If he does then he will get excommunicated from the Catholic Church.
Please correct me if I am wrong.