>>12878698>Sell out his homeland, become a lapdog to a foreign invader
He said it himself in episode 19; ever since he killed his father, he simply couldn't go back. He chose that path long ago, changing sides would mean the death of his father would go to waste.
Additionally, after Lelouch killed Euphemia, he wouldn't joing the OotBK. Although, we've seen him wondering why would Lelouch do that.>and pretty much used every opportunity to kill his best friend who was trying to help him.
When, in season 1? He didn't know Zero was Lelouch until the end. And when he found out, Lelouch had already killed Euphemia.