Snapedom

This was too long as a reply, so I thought I'd post it as a mini-essay:

I think Severus was deeply fascinated by the Dark Arts, in an intellectual way. I think they appealed to Severus's curious nature, and I believe he had a healthy fear and respect of them. I don't think he wanted to be a Dark Wizard or really even use the Dark Arts, so much as he wanted to understand them and be able to defend against them. I see Severus as being akin to the kind of person that would study evil so as to understand it and recognize it and be wary of it in themselves and in others. If Severus was a Muggle, I could see him being interested in becoming a criminal profiler. Although, I suppose he could do that as an wizard, too (by becoming an Auror).

Obviously he was willing to use Dark Arts if the situation demanded (using Sectumsempra against enemies and, begrudgingly, Avada Kedavra to kill Dumbles when commanded). Still, he didn't seem to fling Sectumsempra or other Dark Arts about, willy-nilly (at least not before becoming a DE or after turning against them). Even as a teen and under attack by the Marauders in SWM, he only puts a gash on Jame's cheek (he doesn't unload on him like Harry (unwittingly) does on Draco).

Regarding his attitude to Mulciber's use of the Dark Arts (during the conversation with Lily in the Prince's Tale), I think by this point Severus had become disillusioned. The Marauders were able to get away with cruelty by abusing more neutral spells, so I don't think Severus saw Mulciber's use of a Dark Spell as that different that what the Marauders did on a regular basis.

I don't think Severus was seduced by the Dark Arts, so much as he was a broken idealist. He seemed to have high hopes for Hogwarts (that it would be better/kinder/more just than the Muggle world) and found it ultimately lacking. Severus was still subject to prejudice and abuse, just of a slightly different flavor. And prejudice of other people (Muggle-borns like Lily) was perhaps more rampant than he was led to believe. After all, he thought that Muggle-born Lily would have no problem getting into Slytherin...and he didn't think his half-blood self would have trouble getting in, either.

I'd say Severus probably started to learn about the Dark Arts from his mother's books. I doubt she taught him, as Tobias didn't seem to like magic (let alone Dark Magic). Then he carried on learning about them from books at school and perhaps some of his fellow Slytherins.

I think studying the Dark Arts (not using them) was exciting/thrilling to Severus, and I think his passion for the subject came out in his DADA teaching. I think Harry mistakes Severus's passion and love for a complicated and challenging subject for love of the Dark Arts, themselves.

I think Severus is a person who is very concerned with healthy self-preservation of himself and others (I believe he thinks others should be just as vigilant as he is). He probably thinks that those who shy away from studying the Dark Arts because they are creepy, are missing out on valuable information that may save their lives one day.

Anyway, those are my thoughts, for now. ;)