Kirk's KHAAAAN in TWOK made no sense. he knew he was just buying time for Spock to repair the enterprise and beam them out in 2 hours. Was that just theatrics to fool khan into thinking he didnt have that ace up his sleave?
I think that may be it, yeah. That explanation also makes Shatner look a little better - it's Kirk's over-the-top acting, not the Shat's.
The other thing to consider, though, is that Kirk is really furious at Khan for reasons other than being "marooned" - he's just watched one Starfleet officer commit suicide and he's got a guy who served under him for who knows how many years lying on the floor in a coma, and Khan's responsible for both.