He definitely executed the Cloud Creature without much hesitation, and Spock did much the same for the salt vampire.
He knew how dangerous it was and it clearly posed an on going threat. I believe it was impossible to reason with and was about to spawn, compounding the problem enormously. It was either the creature or many of the rest of the life forms in the galaxy. Kirk really had no choice. It was a matter of "self" defence.
The salt vampire was actually threatening Kirks life when it was killed. So when Bones finally shot it, again that’s self defence or the defence of others. I.e. justifiable.
In neither case is anyone doing it out of vengeance. Though I am not suggesting there aren’t shades of grey in Star Trek.
As it stands, the only reason he didn't kill the Gorn--or for that matter, the Horta--is because both demonstrated at the last minute some redeemable quality; the Gorn had explained the reason for the attack was essentially a territorial misunderstanding, while the Horta had hesitated to kill him and demonstrated itself to be slightly less dangerous than Kirk expected.
The above are examples of where Kirk is "at war" with other species yet makes peace. Not sure how that proves he is ruthless and vengeful? Sorry but you have yet to convince me Kirk would kill something if it wasn’t a continuing threat to himself or others and there was no other reasonable course of action available.
Even after Kruge falls into the larva, Kirks expression doesn’t seem to be enjoyment. Despite provocation it’s more relief.
I think you misjudge Kirk. So far I am not aware of any situation where he has actively killed anything that was both at his mercy and was no longer presenting a threat. Not even if they wanted him to. Besides, Nero never asked to be killed. He just didn’t want Kirks help. A sizeable difference! Anyway, in Nero’s case he is presented as insane. We don’t kill insane people because they say insane things. Kirk needed a better reason than that.
Nobody eh? Thanks, I hold you in great esteem as well.
If hardly anyone cares one way or that other (which is a worry in its own right) why not do it and possibly win over another few percent of your audience?
I agree it looks like that but we don't "know" it because it didn't happen (or are you refering to another example?). True, you get the feeling he would have done it but on the other hand there is not much point in bluffing if it looks like a bluff.
What we can say is that planet had effectively declared war on the Federation. I would not argue that Kirk couldn’t be a hard man. The scale seems over the top but he was working for the future of the planet as much as the welfare of the hostages I believe.
Or tell me if this rings a bell:
Yes, as others have suggested, this might be the writers of that movie making a point at Kirk's expense. Compare it to STXI where they don’t even seem to realise there are moral issues let-alone resolve them.
The Nero/Kruge analogy is valid. Offered to be saved, counter-offer of trying to kill Kirk (Nero still may have had that option),... .
Which is why he graciously let himself be destroyed rather that use it!
No, Nero was finished as a threat to the Enterprise. No-one can seriously doubt he would have tried to destroy it if he could have.
It seems likely that either the Kirk/Kruge or Kirk/Khan resolution was the inspiration for the Kirk/Nero/black hole scene but sadly the writers applied today's morality to it.
By the way, with regard to Khan, he decided his own end. Maybe Kirk could have tried to save him but that would have taken vital seconds to minutes they didn’t have.
)this time instead of the usual lingering shots of sparkles slowly failing to materialize on the transporter pad.
I do agree that transporters don't make much sense though. Dig out the thread on whether transporters are kill and clone machines or not to see lots self-pleasuring on that point... 
