Why did he accept and how is the 4 eyes? And why he didn't kill him?
Why he accepted the offer:
Harold still knows far more about how events are likely to play out in general, and the guy was not only giving him a way past the decapitation thing (which he probably orchestrated) but also a position right up close to his plans, leaving Harold in a prime position to gather even more info on the guy's plans (yeah, I didn't bother remembering how to spell his name).
Why he didn't kill him:
Harold's ultimate goal is to survive while averting the tragedies that occurred in the game's plot, with a big emphasis on
survive. It's not just a matter of "kill the BBEG and be done with it". The guy isn't widely known as the BBEG, so killing him would just be a random murder he couldn't otherwise explain. If he killed him while working for him most suspicion would fall on him due to already being suspected of being a spy - even if there was no actual evidence.
Or maybe he feels that he's not ready to face him, or that he needs the Hero to join him for that battle? I'm honestly far less certain about this one.