It seems like, a strong element of the story is the notion of unintended consequences. We're placed in the almost unique position of seeing the aftermath of one of those end-of-episode morality scenes from TOS: you know the sort, Kirk makes some determination, it wraps up the plot neatly, and the Enterprise and her crew fly off to their next mission, never to look back. This time, Kirk's decision to leave Khan on Ceti Alpha V comes back to bite him in the ass, and has major personal consequences.
But.... to be fair to Kirk's original decision, he was faced with a problem of how do you punish a nign indestructible, super-human whose very instincts are to rule and conquer? He solved that by engaging Khan's very nature, giving him a planet to cultivate and rule over in exchange for going away quietly and not (for example) escaping to try and take over the Federation, or worse. Something that Khan and his followers would almost certainly have tried to do if he'd simply handed them over to authorities for a conventional sentence and imprisonment. Kirk's solution is genius: it appeals to Khan's pride to
"rule in hell" rather than
"serving in heaven". The fate of Ceti Alpha was unfortunate, and perhaps Kirk has some responsibility for not having followed up on and checked that Khan's people were OK, but it
was meant to be a punishment after all -- like the original convicts sent to Botany Bay, there in chains, but with an opportunity to build themselves a better life, if they were to survive and prosper in a harsh, dangerous environment. It was a relatively humane solution, as well as nullifying any possibility that Khan could become a threat to the wider Alpha Quadrant.
Yes, Kirk's actions led to the events of The Wrath Of Khan. But to "blame" him, or ascribe fault, is as simplistic as Khan's bitter recollections of the original episode, when Kirk had actually offered him hospitality and a chance to atone for his past, Khan threw it in his face, and more than that, now spins it as Kirk having treated him badly. Khan holds some responsibility. Kirk holds some responsibility. And simple bad luck / fate, in what happened to Ceti Alpha, holds some responsibility. But Kirk's choice to give Khan a world to 'conquer' was made in good faith, and accepted by Khan as such.
But what say you?