Ask Oswald of Carim
The Gwynevere in Anor Londo is nothing more than an illusion. Would it still be sinful to kill her?

Friend, I know not what thou speakst of. I urge thee to rid thyself of such… delusions, lest thou thyself on a path from which thou canst not return.

If you came across a crime in progress - say, an innocent person grappling with a murderer - what would you do?

Velka rewardeth those to deter the vile and corrupt sinners of this world. However, my lady frowneth upon those who act rashly. Be quite certain that what thou art seeing is indeed what thou take it for. Only then shouldst thou act to resolve it.

If I attacked someone by accident - say, because their round hat was tilted at an angle that resembled an instrument of the deplorable Seath, which happens to be really fun to smash - and they try to kill me on sight from then on, no matter how long I try to avoid them or apologize, would I be justified in killing them to defend myself?

Need I explain to thee that in attacking thine ally thou hast sinned? It matters not what thine intent was, for the world is all too full of good intentions. To make right thy folly, thou needst only speak with myself or another pardoner. It is our domain to undo what is done and mend the bonds that have been broken. Thou needst not slay thy friend, but offer me a… paltry sum and I shall assuage his aggressions.

When I put down or use a summon sign, how responsible am I for who may be harmed or assisted as a result? Should I desert a summoner who has brought me to fight the crossbreed? Is it right to assist a Darkwraith if they are slaying a demon or a monster like Seath? Or to accept their assistance, and allow them to be rewarded for it?

Thou art ever responsible for thine own deeds, be they noble or impure. Likewise, the actions of others are determined solely on their own initiative. Thou must control thyself and thyself alone, and feel the burden of sin shouldst thou be tempted to aid in a deceitful act. However, if a man cometh to aid you, no matter how swathed in sin he may be, it is not thy place to refuse him a chance at atonement.

Do Pardoners absolve the sins of creatures like the serpent men or mushroom people?

Nay, a pardon may be given only to those beings who possess humanity. It is not my place to grant absolution to monsters, demons, or hollows. Even the gods cannot be absolved by a mere bishop such as myself. Humans are fit only to cleanse humans.

Poor Pardoner, yours is a thankless task. I wonder what you know of me.

Thankless? I should think not. My Lady rewardeth with all the joy one could ask for. And besides, the title of Pardoner doth open many a door shut tight to raggedy folk of thine ilk. Art thou a beggar of some sort, then? Begone, unless thou hast a confession to submit to mine ear.

*clings to leg* O-O
Anonymous

Wha? What is this foolishness? Hast thou truly gone mad?

Pardoner, I have two questions for you. Is it considered a sin by Velka to love another of the same gender? Also, have you ever heard of an aristocrat from Carim, who refers to himself as "Marvelous Chester"?
Anonymous

My Lady careth not who one loveth, only that love doth not lead them astray. Affection, friend, can just as easily lead one to corruption as to purity.

As for the so-called aristocrat thou ask of, I have not heard of him. However, the distorted flow of time may offer some explanation. Though this ‘Chester’ doth not appear in any of our old records, he may come from an age after mine own. 

Pardoner, what know you of the Dark Soul? I hath heard Kaathe's words, who would paint Gwyn as a coward, unwilling to allow the Fire to give way to the Dark. Doth Kaathe speak true? Is Dark to succeed Fire?

The Dark Soul? Little I can tell thee of such an old legend. However, heed this warning friend. Thou wouldst be wise to turn thyself away from the old serpent. Thou mayst find the followers of Lord Gwyn to be… less hospitable should they discover the company thou art keeping.

I have a question, Sir Oswald. In my travels I ran into a strange portal... And I found myself crossing blades with the Knight Artorias and slaying him, worse, I refused to give his blighted soul to the Lord's Blade Ciarin... Have I sinned?
Anonymous

The only sin thou mayst be guilty of is deception, friend. At least, for thy sake I truly hope so, for the Knight Artorias hath not been seen for some thousand years now. Even the distorted time of Lordran hath not bridged gaps so great.