An excommunicated person must repent, have the censure lifted by appropriate authority, and then receive absolution in a sacramental confession. In most cases, the priest who hears the confession also lifts the excommunication. Excommunication is not a sentencing to Hell. It is a medicinal remedy to help the sinner to repentance and to preserve the community in good order. Re-baptism is impossible.
Baptism is forever, as are confirmation and holy orders.
Sins committed after baptism are forgiven in the sacrament of penance and also sometimes in the anointing of the sick. Murder is not per se punished by excommunication. No one can incur a penalty in Church law of which he was unaware. Unlike civil law, in which ignorance of the law is said to be no excuse, in Church law ignorance IS an excuse, except where the ignorance stems from culpable negligence. To be formally and subjectively guilty of sin, one must know that one is committing a sin.