The main character certainly has anti-hero traits. And being able to sympathize with him or not depends just on your personal disposition and how pure heroes you need. I've come to really appreciate shows with more realistic main characters like this, and not those bulk can't-kill-even-the-worst-enemy-who-wants-to-kill-you heroes. Thus it's my disposition to easily like Lelouch. And no forcing involved.

edit2: I do not see vomiting 1 day after killing that guy as an emotion. Especially because the editors left such a scene out at the moment of the murder. Which couldve made him a little bit more sympathetic.
Wasn't that when he watched the TV news? Whatever else he might be, he obiously wasn't a seasoned soldier, and likely when he watched those news he saw for the first time really clearly and objectively what the battle meant aside from politics and tactics. Kind of post-traumatic shock. I think it was a good detail, like I wrote earlier; he wasn't yet a true cold-blooded killer. During the battle he probably had enough epinephrine in his system to outweigh any psychological shocks. So, in the long run this isn't a factor making him more sympathetic, just more human. Although those might be the same things...