Glad you read and enjoyed it, though I do think nothing replaces the original text. For starters, not getting the insight into the internal thoughts of the characters allows a lot more room for interpretation. It especially allows more interpretation over why MacBeth gave in and how much he resisted.

While there are people who revel at being evil (when you get to Titus Andronicus, you'll meet Aaron, and in Othello there's Iago), flawed heroes (and MacBeth certainly starts the play as one) tend to be more compelling, personally. Even Hamlet's Claudius, one of his more evil villains is shown to be capable of guilt.

As for MacDuff, it's because he really didn't think MacBeth was evil. Usurping the throne is one thing, killing rivals is another, but going after the family was considered out of bounds.

And yeah, the Witches are such an enigma. Were they simply revealing what was destined? If he'd never encountered them, would he have been tempted? Was their second prophecy meant to make him complacent?

Shakespeare doesn't always use the supernatural; it's just a useful tool depending on the story he wants to tell.

If you're a fan of fantasy, though, I do recommend Terry Pratchett's Wyrd Sisters, which starts with a comparable group of witches and a royal plot, but goes in very different directions.