In that scenario, the “penitent” is clearly not contrite at all and has no real intention of receiving the Sacrament. So the priest should withhold absolution. And if he did attempt to grant absolution, it would be invalid due to the lack of any contrition.
Moreover, an argument can be made that the killer had no intention to receive the Sacrament, and had no desire to repent and be forgiven, and so the conversation in the Confessional was not part of an attempted confession and therefore is not under the seal. The seal only applies to a sincere attempted confession or a valid Sacrament of confession.