Apologies if I've said this before, but Lindsey Graham reminds me strongly of Andrew in Buffy the Vampires Slayer.
As Buffy says, "He's not evil, but when he gets close to it, he picks up its flavor like a mushroom or something."
I came across a fan page about Andrew, and actually find it quite a good guide to Graham's behaviour – explains Graham's volte faces very well. The TL;DR is Andrew is tragic: so desperate to be liked, he murders his own best friend. But when he's hanging around with good guys, he kinda sorta acts like a good guy, because now he's desperate for them to like him.
"I killed him. Because I listened to Warren, and I pretended I thought it was him, but I knew - I knew it wasn't. And I killed Jonathan."
[...]
"Here's the thing. I killed my best friend. There's a big fight coming, and I don't know what's going to happen. I don't even think I'm going to live through it. That's, uh, probably the way it should be."