Does Sandor really enjoy killing?

I agree with eyesofmist's post. If Sandor did enjoy killing, you would think he would seek more opportunities to kill when he was running wild, with no master, no loyalties, nothing to lose. He did not though. In fact, he spends this "free" time saving a little girl and helping build a wall to protect a village. He kills not a single member of the Brotherhood, even though they had taken away every penny he had. He does no harm to the ferryman. He does not kill the pig farmer from whom he steals the cart and clothes to get them into the Red Wedding. Any killing he does at the Red Wedding is purely for survival, and he takes the time to save Arya from going to a certain death. The one man he kills who was not killed for survival, he kills "tenderly" and with mercy.

I thought your idea of Sandor having a kind of suicide wish is likely true. I think as I've been rereading the text, that Sandor seems to advise others according to what he himself desires. Eyesofmist analyzed that scene where he tells Sansa to smile, be sweet, be Joff's ladylove as being what Sandor himself would want. I think that's a really valid reading. He also tells Sansa to save herself some pain and give them what they want, which is exactly what he appears to do. In Winterfell, he expresses a wish for Bran to die faster--wishing the maimed, 7-year-old boy a merciful death is something I could see Sandor wishing to have had for himself, when HE was a maimed 7-year-old boy, betrayed by his father and brother. Even when he goes to Sansa's room and pulls the knife on her, Sansa thinks he means to kill her. Sansa is good at picking up Sandor's intent. She was not afraid when he pulled the longsword on her just a few days before. They were all alone on top of the holdfast and he laid his sword on her neck. She did not fear him then. Whatever was going on in Sandor's head the night of the Blackwater, and there was a lot going on, I think one part of him was probably going to give Sansa the mercy rather than leave her to Joffrey's cruelty and/or the horrors of the invading soldiers.

True to the nihilistic themes that surround him, Sandor wants to die. He is the butcher, the Stranger personified. He rides a black horse and brings death. I'm thinking back to other novels which show this existentialist crisis--The Stranger (The Outsider, for those outside of America , The Plague, Death in Venice, The Metamorphosis--these all have characters who only wake up to life after facing death, and there is often a morbid fascination with death (at least in Mann's work). To oversimplify, there are basically two ways a character in a crisis such as Sandor's can go--they receive the epiphany before they die of what life is really about (perhaps finding meaning in their final moments, like Merseult, perhpas) or having faced death, they are invigorated to live, having a new appreciation for life, shaking off the dulling apathy of existence to live, truly live, until they die (Ah! Ygritte! There you are!).

The Hound is dead. Sandor Clegane is at rest--facing the consequences of death every day as the Gravedigger (I'm again reminded of Camus's the Plague, which had our characters doing a similar service for the plague victims). I have to believe Martin is setting him up to come back to life, full of sap and eager to live.

Regarding the killing of Mycah, as you say, we find out later in the narrative that Sandor laughs at really inappropriate times. Even when he is seemingly dying of his wounds at the inn, bleeding out, he makes a joke--in the immediate aftermath of killing The Tickler and everyone else--he jokes that he wants to see Arya marry Sweetrobin. Then he tries to laugh but it turns into a groan. It's the last time the Hound tries to laugh so far in the series, and he can't quite get it out. The killing of Mycah, a young boy playing at being a knight, could be part of the death wish. Remember that he calls himself a butcher. Mycah is the butcher's boy. I wonder how much of the enjoyment of killing comes from imagining himself being killed, being put out of his misery.

So, to some degree, does Sandor enjoy killing? Maybe, if he's thinking of it as putting people out of their misery. I think on some level, too, though, he's probably taking pleasure in doing his job. Being efficient and the best that he can be. I think, despite the little idealistic boy that's still there somewhere, he does believe in "kill or be killed," and "might makes right." Before Sansa, maybe killing was a way of establishing himself as top of the pack, which for someone as hurt as he was could be almost comforting? I don't know. This is a lot of speculation and hard to prove.

I will say, when Sansa sees his face "transformed" I suspect it has something to do with the fact that for maybe the first time in his life, he is killing for a noble purpose--to save the fair maiden. He is the ultimate killing machine, and maybe there is a sense of being a berserkr there (which for Sansa, who admires his ferocity, might be rather attractive, especially given the circumstances). I wonder, though, if he were transformed because at that moment, he truly was the hero, saving the girl, being what (we suspect) he always wanted to be. And then the fair maiden forgot to thank him...poor Sandor.