I dont blame Sylvanas. She never should have left the shadows she was best in. She was evil but she had her little hidey hole she could hiss out at the world from and we all gave her her space and it worked.
I blame the writers for putting her in the spotlight and basically forcing her to do things and the more they pushed her to do things the worse it got all while they gave themselves pats on the back for how good a job they thought they were doing.