I agree with you, Kim, I think she had a lust for power. Informing on her own husband gave her a taste for it, and for vicious, underhanded court intrigues in general. I'm sure that arranging Katherine Howard's assignations with Culpepper gave her true emotional satisfaction, the illusion of total control and illicit power over others, one of them the queen consort of England. But she was in over her head, and she paid the ultimate price - first her sanity, then her life.
If one were really cynical, though, one might suspect that Lady Rochford, having been the undoing of her husband and his sister the Queen, had in her sights the undoing of yet another Queen, related to the Boleyns by blood... Only, perhaps this time around she underestimated the degree of her own complicity, perhaps she mistakenly believed that she'd escape justice... Perhaps indeed she was playing a double game. The woman is a genuine mystery, a true psychological enigma.