It's definitively not intentional when his friend die, but it's still his fault for involving them, directly or not. CC warned him that similar thing would happen if he stayed close to the one he loves, but he ignored the warning and thought he could continue living like he was before.
The only time Suzaku was blinded by rage was at the end of the first season, and it lasted until the mask fell off. There is no way Suzaku would know what Lelouch is really thinking, when Lelouch never show his feeling. The guy still pretend to not be involved in anything, and never show any remorse. What other conclusion could Suzaku draw from this other than Lelouch being a dangerous person?
I agree when you say Suzaku is getting darker, but it's nothing surprising after losing another friend because to Zero. His "dark side" manifest in his growing obcession to catch Zero, but he is still fundamentally the same.
"All the evidences.. and the testimonies say Lelouch is innocent. But my heart has always believed Lelouch is the suspect, that he is Zero. In order to prevent any more tragedy, I will take on any method."
Is he really twisting Karen's arm? He isnt lying or trying to trick her, he asked her to do what she believe is right.