She's a complicated lady. Yes, she married her dead husband's brother, and yes, her new husband murdered her old one. But it's never quite clear how much of the crime she's in on: she never owns up to it, and she has more empathy for her tortured son than anybody else in the play. While everybody's trying to figure out what's bothering Hamlet, she's the only one willing to state the obvious: "His father's death and our o'erhasty marriage." Well, yes.
So what's she doing on this list? The special horror of Gertrude lies in the fact that she truly feels tender, maternal love for Hamlet — yet she betrays him anyway. One minute she's all compassion, the next she doth protest too much. And she's an incorrigible flirt, the original Elizabethan MILF: no wonder Hamlet could never settle down with that nice Ophelia, when his mom won't free him from her Oedipal apron-strings. "What have I done," she asks her sulky son, "that thou darest wag thy tongue/In noise so rude against me?" Think for a minute, Gertrude. You'll get there.