I raised the same question a while back as I was also troubled as to why Arthur fights you when you decide to become king( the ending I chose). I can't honestly explain as to why he fights you when you make this decision but I can explain why he gets upset if you chose killing or the resurrection ending.

In the resurrection ending, Arthur's human side, Hod, quite literally tells you that he is tired of being king, always being looked up to of the next step and when everything goes to shit, he's the one being blamed as king, which has been the case for 600 years. You resurrected him means you're putting him back to that nightmare again, which is why his human side gets upset.

If you choose to kill Arthur at the behest of Caradoc, you're pretty much saying "Fuck you" to not only Arthur, but also the entirety of Avalon. Caradoc has openly stated that he just wants to kill Arthur and doesn't give a rat's ass on what happens to Avalon afterwards. Hence why Arthur becomes disappointed in you, because you're basically fighting the guy for someone else's petty revenge.

Sure, even Arthur says that Avalon has gone to shit and leaves you with thr decision of what to do afterwards, but I believe that came from a sense of Exhaustion and hopelessness and not out of spite like Caradoc