The classic example is Darth Vader/Anakin Skywalker and his son.
Darth Vader is a terrible man who does terrible things. Nearly all of his influences are evil. Emperor Palpatine encourages him to embrace the darkest depths of his despair and anger and lash out with them against the world. He started out all right -- a little boy who loved his mother very, very much. Somewhere, though, it changed. He lashed out with anger, and somewhere along the line, the Jedi must have rejected him, one by one, or in one fell swoop.
Somewhere, he crossed the line, went over from forgiveable offenses to a place where each of the Jedi he didn't kill (or hadn't killed yet) had to say: "You are beyond my power to help. I can do no more," and leave him to stumble back to redemption -- or not.
Holding out hope for those who have passed beyond the point of no return is the definition of a saint, and a masochist. I suspect the difference between the two is slight, and depends on perspective, and how bad the injuries are.
And who defines the 'point of no return'? Yoda was convinced that Anakin was already edging beyond it even before he pulled his little stunt on Tatooine. I think Yoda gave up first. Odd -- I'm suddenly equating Yoda with Dumbledore, with all of Az's love and affection.
Diane Duane balanced it best, of the modern writers, I think. "Fairest and fallen: greeting and defiance." One does not snub the Lone Power when one meets It. Yes, evil. But no need for you, yourself, to become rude about that. I think that's where the Puritan-era Christians may have screwed it up. One does not cast the bathwater out because a little bit of the Devil got in. One acknowledges the presence of the le-matya in the living room, and goes about one's business taking the proper precautions, which may include removing the le-matya, sensibly -- and carrying a stunner when walking in le-matya country. Jesus hung out with hookers. Who are you, then, to doubt your faith so much as to cast out all those who don't conform to your way?
Everyone of the light repudiated Anakin Skywalker, before the last, and thus he became Darth Vader. He lost faith in himself, in his own will to be good -- perhaps in large part because no one believed he could. It was only when his son Luke told him that he still had good in him that he was obligated to try to prove Luke right.
Will I, someday, be Luke Skywalker to someone's Darth Vader? Will I be someone's last, best hope?
It's said that you can tell who your real friends are when you start having problems. Your real friends are the ones who will stick with you, and give you the hugs, the love, the sympathy, the harsh reality, and the boots in the ass, all as appropriate. They're the ones who will say, "Why the fuck are you doing this to yourself?" -- and you don't hurt them for it, because it was what you needed to hear right then.
And there are going to be people who see you turning towards things darker and darker -- not necessarily dangerous to others, as Anakin did, but dangerous to the self nonetheless -- and give you warning, and then when you slip further and further, lost and unable to find a way back towards love, they will leave you in disgust, because no one wants to get sucked down with a sinking ship.
They may have been your friends, or they could have been. It's always a temptation for the would-be saint to count those as failures, where the necessary deep connection needed to yank someone out of those depths wasn't made, couldn't be made.
Where do you draw the line? When is it right to say, "I've had enough of your bullshit. Call me if you ever figure out how to be human again."? When is it right to say, "I'll be here for you, no matter what," and follow through, in the worst of times? When does the friendship outweigh the darkness that comes up?
I heard an illustrative story, once, probably from FatherSir, about the differences in friendship, and what's called friendship. It was a conversation between a Russian and an American.
"I have two friends," the Russian said.
"Oh! So few friends!" the American exclaimed. "I have at least twenty!"
The American proceeded, after a while, to have the sort of problem that involves a crisis at three o'clock in the morning.
The Russian hears of it. "You should have called one of your friends," was the comment.
"Call one of them at three o'clock in the morning?" asks the American, bewildered. "That would not have done at all. That would not have been polite. They would not have understood."
"They are not your friends, then," the Russian says. "I am lucky. I have two friends. I can call at any hour if I need to, and they will understand, and I will understand when they call. Not everyone has even one friend."
That's the definition of friend that the saying talks about, with the wisdom that when one has a crisis, one finds out who one's real friends are.
And it's hard to be friends with someone who's demonstrating that they're not walking on the light side of the Force. It's hard to be friends, when someone has been listening to the Lone Power, and stops listening to reason...
... but what if you're their last, best hope?