TwistedBuscuit. Huh. You don't happen to be related to TotalBiscuit?
OT: Killing Henselt IS the correct thing, the right thing, the morally thing to do. Fuck Henselt. And fuck Kaedwen as well. Henselt is vicious, to the point and confident. He's just a badass whose ambition stretches far, although not limitless like Radovid who, if given the opportunity, would probably take over the world. Regardless, Henselt is an unethical bastard and making sure he's dead might not be good for Kaedwen, or the Northern Kingdoms in general for that matter, in the short run but I definitely think it would be for the best in
long run.
We're told by Geralt when we spare Henselt that Kaedwen would prosper by the ruthless rule of their tyrannical king. Is living in fear and poverty really a valid substitute for anarchy and freedom? Obviously, this is up to you. But I can't in my good consciousness let such a vile of a man live. I'm not a utilitarian, in fact, Henselt proves my point perfectly as to why I hate utilitarianism so much. The fact that you can sacrifice the happiness of some so that the majority can prosper is wrong. Henselt should die because the Kingdoms will do better without him. His bloodline has degenerated into a corrupted mix of poison and hatred. That kind of negativity just doesn't suit the role of "King". Radovid can annex Kaedwen for all I care. Anyone is a better ruler than Henselt.
Obviously, the whole revenge aspect is a compelling reason as well. Even if I'm not a big fan of the Blue Stripes, seeing them die really hit hard. I prefer Iorveth over Roche, without a doubt, but he's a good man nonetheless and seeing him lose his family hurt. Then there's Ves... That alone was enough motivation for me to kill Henselt. Nations and ethics be damned. I'm not interested in preserving the future of one man when that said man ruined the future of others.
Besides, I don't give a shit about the Northern Kingdoms. I really don't. Most of the citizens are stupid, inbred, racist plebs. Fuck 'em. What I do give a shit about however is the new realm that welcomes all races - ruled by a dragon in disguise of a fair maiden. Saskia's Upper Aedirn is a nation worth fighting for. It really is. It's the reason why I let Stennis live, that coward piece of shit of a "prince". I didn't want to taint the sense of justice the new state would have. I didn't want to set a "mob rules" precedent. They're good people and it's a nation that can prosper only if they all set aside their fucking differences and work together, Vergen could florish to become a moderately awesome state. All that accumulated knowledge from different perspectives the races bring to the table coupled with the wealth of Vergen enables the people to reach success and independent sovereignty.
Once again RED proves how awesome they are. Here we are, discussing what the right thing would be to do when there is no right thing to do

I'm not right, neither is anyone else. All we can to is explain our motivations and hope others can empathize. I don't really "roleplay" in the traditional "supposed to" sense. When I play roleplaying games, I like to put myself in that world and then ask myself "what would I do if I was put in this environment. So, that's what I would've done if I had a chance to slay a king. It's interesting to read peoples perspectives though. Both people who play as themselves, from a lore-specific Geralt and to a book-wise Geralt. It's good stuff. Adds a lot of depth to the discussions. Kudos RED!