Well, she is insane, not an idiot, or at least more of one than usual for a bigot.Huh. That just made me realize that, at least in the movies, Bellatrix is the one who initially demanded an unbreakable vow from Snape and read out the conditions (it's not clear who decided on the wording, but she might have done that as well). She also displays doubt in Riddle's judgment in the preceding scene, so she's not 100% blindly loyal to him.
If I remember correctly, she was very suspicious of Snape and questioned his loyalty constantly. It maybe why she went along with Narcissa's unbreakable vow plan.