Okay, so, this takes place in an alternative timeline where Harlow hates Gaster, not as much as he does in usual DF though. In the case of it all, DF3 Harlow still believes he's living a lie, and thus, due to the fact that Chara, Asriel, and Sans are there, he cannot take it anymore. Now, Gaster doesn't have control of this timeline (yet) despite it being in the Void, and thus he cannot control his son. Now, this mostly focuses on Week 2, which can be considered the most canon thing in Five Nights at Harlow's, as in Week 1 we don't really see his hate for Gaster or anyone in general. Due to that, this lore will mainly be focusing on Week 2 due to Week 1's irrelevance. In technicality, Harlow went insane due to the fact he has been through so much shit he can't take it anymore (understandable). However, this also proves my theory correct on the "black tears". They are indeed from Hate, and what also proves this is the line behind Harlow in one of the endings saying "These thoughts aren't my own", and since Hate takes over one's freedom/sanity/control for power, this ties into the "thoughts" that aren't his own.
(this is a copypasta due to a discord conversation)