Rape is treated differently to other crimes, so that's not a relevant example.
Person A has killed person B. Person A says it was reasonable force in defence. Do you just take their word for it? Do you make a judgement based on age, sex, "race" or whatever? Or do you investigate? If you investigate, do you leave the killer there while you investigate, free to do a runner if it turns out they were lying to you? People do sometimes lie to the police.
Someone I know, let's call them X, was once found by the police (responding to an emergency call by a third party who was of course not identified) in a house with a machete in his hand and a person bleeding on the floor from what looked rather like an injury from a machete. X refused to drop the machete when told to do so by the police and refused to go to the police station to make a statement. The police arrested X. The initial investigation corroborated X's story - the wounded person attacked X with the machete, X disarmed the person attacking them and that person was wounded during that. X was then released without charge and their actions recorded as reasonable force.
Should the police have just left X there, machete in hand, and gone home solely because X said it was defence?