"'This brother of yours, he was possessed by the devil.'" (1.63)
The question of what happened to Louis's brother is a question of good vs. evil. Was he seeing visions from God or from Satan? This question is also unanswered, just like Louis's good vs. evil dilemma. Also, if Satan exists, then can evil be attributed to him? Are vampires (or people) evil by nature, or because evil influences them?
"People who cease to believe in God or goodness altogether still believe in the devil. [...] Evil is always possible. And goodness is eternally difficult." (1.67)
Can you have good without evil? If evil is always possible, does that prove the existence of good? And does Louis ever see any good in his long, long life? Is Louis better than Lestat and other vampire precisely because he's so tormented? Goodness is supposed to be difficult, after all.
"'I'd like to meet the devil some night,' [Lestat] said once with a malignant smile. 'I'd chase him from here to the wilds of the Pacific. I am the devil.'" (1.169)
Trying to follow Lestat's circuitous logic, would he be chasing himself? Disregarding these confusing statements, what's evident is that Lestat seems to be evil for evil's sake. He enjoys it. Maybe doing evil acts and enjoying it is what really makes a person evil.
"She said, 'Get thee behind me, Satan.'" (1.287)
Babette Freniere is not talking about a White Stripes album; she's calling Louis Satan. As if Louis weren't already conflicted enough, now he has a woman whom he loves and trusts call him a devil—and she really believes it. Because he takes his religion seriously, Louis will never get over this, later saying "I don't know whether I come from the devil or not! I don't know what I am!" (1.302).
"The question pounded in me: Am I damned? If so, why do I feel such pity for [mortal Claudia], for her gaunt face? [...] I felt, yes, damned and this is hell, and in that instant I had bent down and driven hard into her soft, small neck." (1.321, 1.322)
After being called Satan, Louis gives in to his evil side. We don't think he would have killed young Claudia if he were not having such a moral crisis.
"'Evil is a point of view,' Lestat whispered now." (1.382)
It's all about perspective. From Lestat's (essentially evil) perspective, Louis isn't being evil. But from Babette's perspective, vampires are evil, fair and square, and no matter what Louis or Lestat think about the matter, Louis's actions are evil.
"I grabbed him on the very steps to the Communion rail and pulled him down to face me there and snap my teeth into his neck." (1.647)
Well, if there was any question as to whether or not Louis is capable of evil, here's your proof. When Louis devours a priest in the middle of the church, it's pretty clear that he is capable of unholy acts.
"It struck me suddenly what consolation it would be to know Satan, to look upon his face, no matter how terrible that countenance was, to know that I belonged to him totally, and thus put to rest forever the torment of this ignorance." (2.6)
We've heard that there is a difference between what is right and what is easy. (Thanks for the quote, Dumbledore!) This implies that "easy" is bad. The easiest way for Louis to put his spiritual torment to rest is to side with evil.
"'For all I do know... [God] doesn't exist.'
'Then no sin matters,' [Armand] said. 'No sin achieves evil.'" (3.164-3.165)
Vampires aren't evil. Or maybe they are. Armand's roundabout logic answers nothing, but he is very good at talking in circles. Is this kind of roundabout excuse-making actually evil?
"'Your evil is that you cannot be evil, and I must suffer for it.'" (3.309)
We're back to talking about point of view here. Claudia, who is arguably evil, thinks that Louis is evil because he refuses to be evil, which ends up punishing her, because she needs to commit evil acts to survive. Clear as mud? That pretty much sums up how complicated some of the moral issues in this book really are.