Why exactly did Horus accept Erebus' bullshit???

I finished the first three books of HH but it never really made sense why Horus chose to side with Erebus. Horus was fully aware that whoever it was that came to him was only pretending to be his dead friend, Horus had no way of verifying whether his claims were true (and considering him lying about his identity there was no reason to give it any credence), Magnus the fucking Red projected himself across the galaxy in violation of Emp's rules and told Horus to not believe that bullshit and Horus somehow still sided with the bastard. I would understand if he was delirious at the time and wasn't thinking straight, but that was clearly not the case. I'm sorry, that whole thing kind of doesn't make any sense.