In the end, there is no universal definition of heroism or villainy. Which is why we can debate this.
I believe we shouldn't be dismissing several of Lelouch's motivations far too easily. I would certainly consider him to be self-centered, in a sense, but he did explicitly and implicitly want to make the world a better place. That wasn't being "pretentious" as you have claimed, since he did take various steps towards the goal. The fact he also had other concerns and interests in mind doesn't change this. Beyond the mere acts of cruelty you mention, he did try to help other people even when it didn't necessarily serve his quest for revenge.
It is true that Lelouch has to deal with a number of moments of doubt, when his most personal feelings are involved, but that doesn't mean his actions have no meaning in general. It means he is still a conflicted young man and not a perfect or mature individual, but rather someone who must face challenges and undergo development in order to overcome both his internal and external obstacles. Which he does.
I also think your interpretation of the ending is highly subjective, slightly inaccurate and widely open to debate. Lelouch was always aware about the questionable morality of his actions, so that wasn't something he realized only at the end. There was a need to atone at work, yes, but also a desire to actually change the world. This cannot really be denied, even if you might not agree with how the show presented it. Calling it escapism is possible but nevertheless far too simplistic. Beyond that...what is or isn't considered sad is always going to depend on the perspective of each member of the audience and how they connected to the character(s) involved.
Kiritsugu Emiya is a very different character and not the only possible version of a dark hero, so the direct comparison is of limited use. Among other factors, he is both older and more mature, in order to explain why his resolve is greater, as well as why his emotions are more reserved. Based on their respective personalities and situations, Lelouch isn't going to be like him.