Okay... so let's take this to its logical conclusion. Let's say Luke manages to cut down the old unarmed man in the chair (as far as he knows at that time) and then kills Vader...what then? Does he call through to the Imperial officers and say "Hey boys, I've killed the Emperor and Vader so will you stop the attack now?" They're going to be all, like "Yeah bro. No worries"? More likely "Are you kidding, we've got this rebel scum exactly where we want them. Thanks for the heads up, we'll use the full power of the fleet now." Luke's actions will make no difference except to sate his own anger and hatred - his own feelings of impotence/helplessness. (and why I keep trying to point out that the two battles are separate; the rebels win the battle against the Death Star, and missing that aspect confuses the issue of what Luke's choices amount to) Giving in to his own hate and anger would make him no different to Vader, whose turn was (read; was supposed to be) based upon the same basic error. While you suggest that it is absurdly sentimental I would suggest that it is about whether the means justify the ends, or rather - whether the ends are what, in anger and hatred, you think they are. Oh...and as for the sail barge; what did Luke blow the sail barge up with? The large and dangerous gun sitting atop it. He destroys the sail barge because it still poses a risk to their escape.