As far as Suzaku goes, I think Zan Sidera of the Rockman Perfect Memories forum has the best analysis of his character that I've seen:
(responding to Suzaku flaming)
@Zan:
You should be well aware that Suzaku threw everything out of the window when it came to dealing with Lelouch, due to the Euphemia incident, everything to stop Zero. And he's well in his right, for as much as he's aware, Lelouch has repeatedly lied to him about him being Zero, about Geass.
Then used the woman Suzaku loved, Lelouch's very family, a person Lelouch grew up with, the woman that only wanted peace and gave her all for the japanese, to in a grandiose act of betrayel kickstart his Black Rebellion by turning her into a mass murderer of all japanese, violating her free will, destroying everything she stood for, her immortal dignity and status trashed, and subsequently killing her in the name of his United States of Japan.
As far as Suzaku is aware, Lelouch committed the ultimate sin toward the woman that he loved. That's why it's only natural that he'll do everything to stop him, even if it means lowering himself to Lelouch's level.
However, that doesn't take away that Suzaku needs to wake up an act on the realisation that this is not what Euphemia would have wanted. That same realization that is holding him back by hiding his own darkness from Nunnaly. It is that same thing that drove Lelouch into depression a while back, learning that Nunnaly opposes him. Which is pretty much why Lelouch and Suzaku are now exactly alike, driven by revenge, fighting for women that wouldn't even agree with their means.