I think L.L lied about everything to suzaku because he really doesn't want to hurt suzaku. By simply accepting every sin and making suzaku feel justified in his actions he's protecting suzaku in a way by giving suzaku someone to hate. If he had explained that the whole euphie incident was an accident suzaku wouldn't have anyone to vent his hatred at.
Also as L.L. was trying to get suzaku to save nunnally rather then join his side it didn't matter if suzaku liked L.L. As long as suzaku felt compelled to save nunnally. While if L.L had tried to explain away every action it could have looked as though he was trying to manipulate suzaku.
It seemed as though suzaku realized L.L. was lying possibly the same way suzaku's father lied when he was dieing. It'll be interesting to see what suzaku does.
Thats my take on why he lied at least.
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I think that's partially true as to why Lelouch lied again. He in a way did try to protect Suzaku at that moment, just like he tries to protect those he cares for. But it's also a trust thing. Bottom line is, Lelouch doesn't trust him with the information. He's assuming responsibility for everything, and that's part of the message I think this show has been giving us. By lying, Zero has gained what he needs to finish what he started, but he has completely separated himself from everyone he cares for, and because of that, has lost so much in return. For once, Suzaku had the clarity to see that.
Lelouch may want Suzaku to believe he is the evil sociopath that he's hated for so long... And Lelouch may tell himself that he can take responsibility for everything, and beg, bow down... because Lelouch thinks it would be the easiest route to getting Suzaku to do what he wants him to do. (And it was, Suzaku was going to protect Nunnally. Lelouch was right, and really there will always be some sort of manipulation behind his actions.) But we all know there's more to his reasoning than that... Through lies, he would have Suzaku believe all his hatred is justified.
However, the fact that Lelouch showed such surprise, and such a reaction to the "betrayal" also tells me that of course he cares. Lelouch has an enormous amount of empathy, I think that is why he can manipulate people so well.
Lelouch has learned to only rely on himself, so if his "plans" go wrong, or if anything is accidental, he loses control, he loses that power. He didn't tell the truth because he didn't trust Suzaku with the information, plain and simple. So admitting that he didn't give Euphemia that order is a pride thing as well, to tell someone would be placing the burden he carries on someone else, and he only trusts himself to carry it at this point, no one else... Ultimately/sadly, everyone is a pawn. Period. Lelouch thinking Suzaku has just betrayed him has done nothing but solidify that.
Also I want to point out one thing too that I think people have overlooked, Lelouch didn't even tell C.C. it was an accident with Euphemia. He acted as if he gave her that order, as if it was all part of his plan.
If Lelouch tells everyone that Euphemia's death was rooted in him accidentally giving her that order, in a way, she will have died in vain. People believe she was a racist bitch who slaughtered Japanese, they believe in Zero, they believe they are justified. People find out she was Geassed... She becomes a martyr. The bottom line is, during a time of war, people need to believe in good guys and bad guys... Zero is an extreme. That's why they believed in Zero... You're either over here... or over there. Britannia <--> ghetto.
The bottom line is, Lelouch wants to control everyone, for their sake and because he doesn't trust them to do anything right.
Also I thought it was really interesting how Lelouch/Zero did the intro, not C.C.... That was ahhhhhhh!!!!
Also this whole lying thing with Lelouch, Sunrise really has a taste for plain black and white irony. Charles wants to rid the world of lies lol and in that spawned one of the biggest liars and manipulators in history (besides himself that is)
imo very interesting isnt it?