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.
"Blood in the gutter is how they butter their bread at the top."