This is the most explicitly defined Bad Thing To Do, because it's been discussed in so much detail in the books. The First Law comes a close second but is less precisely defined; all there is is the "it's like adding
I am a murderer into your psyche and letting it take root" from SF.
The Second Law ultimately stems from both of these, and so is clearly Not Good for the same reasons.
The Third Law is initially more about a 'social' rather than 'spiritual' danger. To me, "merely" reading minds doesn't automatically stain your soul, for lack of any better way of describing it; it's more about accepting the easiness of it and the sort of person you become as a result of doing it more and more. Like the First Law, but without the killin', but still a slippery slope to the Black.
The law on necromancy (the Sixth?) is much the same, to me... embracing necromancy leads to thinking of live people as potential useful dead people, and that, again, is Not Good.
Contacting Outsiders carries the suggestion of SAN loss... and so doing it would be A Bad Idea.
The ban against time travel would seem to carry the least amount of "spiritual backlash" with it... if, of course, you consider the potential to destroy the entire universe no big thing. The Law in particular seems far more about good sense than turning you to the Dark Side.
And there's my brief Law-shaped nutshell