Sylvanas made a pact with the Jailer, after falling off of Icecrown. The pact damaged the normal balance Souls->Arbiter->Covenant. If Sylvanas hadn't made the pact, the balance would have been preserved. Or in other words: if Sylvanas hadn't been that selfish, everything would be good.
The only logical conclusion: Sylvanas is evil. Not that hard to figure out.