If it were just in her nature then she would have been this way from the start. She wasn't.
Dunno. WoWwiki with source reference seems pretty clear upon the idea that Banshees suffer through immense pain and have a desire to release pain unto others as a result. Having free will, but the conditions of your existance being bound to some magical rules, won't make a nice person out of you.
'Many people die at twenty five and aren't buried until they are seventy five.' - Benjamin Franklin