Light has some pretty rigid moral code. There's a possibility that Lelouch's ideology would conflict with his. If you ask me, Lulu's WAY more human than Light, but he wants to destroy a whole society and drown it in chaos, so a better one can be made. Light is all for making things more orderly and 'improving' the current society so people will be nice to each other. Though of course he's no hinderance...Light wants to dominate the world *philosophically* and Lelouch---doesn't even actually want to rule the world, afawk, he just wants to bring it downs to its knee *physically and socially*. That's quite a livable condition. Lelouch, on the other hand, might want changes to be drastic and break off from Light. At least that's how it could be marginally possible..? XD
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