Would having powers/using magic in a place like the marvel universe still be considered a sin?

Answer 1

Doctor Strange. Enough said.

Answer 2

Sin is a word that attempts to ascribe mythological weight to a moral wrong.  It's a fake concept.

Having powers/using magic is neither a moral wrong nor right.  It's the act performed that gives it moral weight in either direction.  And perhaps how safe/risky it were.

Answer 3

I think both marvel and dc have done stories where its like, you're in the clear as long as you pay your debt, every spell has a cost and as long as you don't use evil ones that cost a human soul and only use good ones that cost, at worst, an eye of newt, then you're not a sinner.