Sylvannas was always a terrible person after becoming a Banshee. That wouldn't be "bad writing" done only recently.
You people in these threads speak as if she had a "rigthful claim to Lordaeron". You forget that Grand Marshal Garithos, despite being an annoying, racist fuck, was actually a Knight of Lordaeron, led an Army (That helped Sylvannas defeat Balnazzar to reclaim Undercity for his people) and then...got betrayed and murdered.
Just because she wanted it for herself.