Beautiful, deceitful, and manipulative, the Queen will stop at nothing to see her son become king. You've heard the rumors about all stepmothers being evil. It turns out, so has she. The Queen tells Imogen that she won't be like the "slander of most stepmothers, / Evil-eyed unto you" (1.1.81-82). Right.
We're so calling her bluff. The Queen totally lives up to the evil stepmother stereotype by actually trying to kill her stepdaughter and those close to her. When poisoning Pisanio doesn't work out, the Queen settles for killing Imogen.
In the end, the truth comes out. She confesses on her deathbed that she never loved Cymbeline, she tried to kill Imogen, and oh yeah, she was going to poison Cymbeline, too. Talk about evil. She tries to control Cymbeline—pretty much by means of her smokin' bod—purely to get power. When she dies in the end, it seems appropriate: yeah, she's missing out on the family reunion, but that's because she pretty much tried to kill of the entire family.
There's no complexity to the Queen. She's as evil as they come. Because her evil comes out of a hunger for status and power, Shakespeare leaves us to question what exactly status and power are—and what it means to want them.