I feel like Oliver having been his therapist and kinda liking him, probably didn't report him due to liking him and also sympathising due to his backstory, which is weird normally but ig people are blind in love. I do agree with you, should've reported him.
tbh it was strange from the start, what do you mean to tell me that you found out he was a serial killer and then wanting your ex to interview him?
oliver should have reported alex if he confessed into being a serial killer but no, he decides to introduce him to his ex?
it all felt off