There's a trope I like a lot in fiction when it's done well: characters who are so ardent in their pursuit of their goals that they end up using means that are antithetical to their ends. Often, for example, this appears in the context of people who are staunch advocates of freedom but who find themselves in power and end up feeling forced to institute totalitarian measures. Or, for another example, people who are upset about the inhumanity of androids end up treating the androids inhumanely.
It's such a common approach as to be almost a cliche, but for me it has a lot of resonance, when done well.
But for it to really work for me, I have to be willing to believe that the characters really are forced into the reprehensible actions; I have to really feel that, given who they are and their circumstances, they have no other choice. (Or, in some contexts, that they're carried away by emotion and aren't thinking clearly about the implications of their actions.)
And too often, the author doesn't give me quite enough to make me really feel that there's no other choice. Fairly often, I'm intellectually willing to believe that the character (for reasons that haven't been made clear to me) really feels there's no other choice, but in many of those cases, I don't know enough to really empathize with that feeling.
A specific example: I'm still enjoying Tigana tremendously, but I've reached a point where a protagonist does something that seems to me to be in opposition to their ideals, and although there are some justifications presented afterward (and although the protagonist feels very bad about doing it), imo the author didn't lay enough groundwork ahead of time to really get me to believe that this was necessary, and worth betraying their ideals. I suspect that part of what's going on is that the protagonists' ideals are not what I've been assuming they are--the protagonists are very much ends-justify-means people, and I'm used to protagonists who have noble goals being strongly opposed to using impure means. So I'm sure it's in character, and I'll probably end up willing to believe it's justified. But at the moment, it's bugging me a little.
(Wrote most of the above a few days ago. I've gotten further in the book and am now feeling that the action in question is a little more justified, but still feel that the author is failing to make the relevant counter-argument to the protagonists' justifications as strongly as I'd like. But it's not detracting much from my enjoyment of the book.)