Lady Macbeth is an accessory to murder, but she is not the murderer, and cannot, therefore, be more culpable than Macbeth, who did the deed with his own hand. That she is an accessory goes without argument: it was she who drugged the guards and who laid the daggers out in readiness; it was she, also, who rang the bell to alert Macbeth that the preparations had been made.
But she was no murderer. As evil as Lady Macbeth is, she couldn't actually bring herself to commit the act (she said she might have done it, had Duncan not looked like her father as he was sleeping). So the Lady is all talk, no action.
Where people would argue that her culpability lies is in her talk. She is brutal in her manipulation of Macbeth, drawing on every possible strategy available to woman. The argument for finding Lady Macbeth blameworthy hinges on the fact that her "motivational speech" changes Macbeth's mind. Completely.
Just prior to this speech, Macbeth has decided that he has no good reason to kill Duncan (he hasn't), and he has just informed her of his decision to "proceed no further". She responds to this by saying that if he can lose his ambition so readily, his love for her must also changable. Then she insults his masculinity, saying he must be a coward (afeard to be the same in thine own act as in thine desire). He tries to tell her that he dares all that a man should dare, to which she replies that only if he dared to do it would he be one. At each stage in her argument, she increases the brutality of her verbal attack until (my favorite) she says if she had made a promise to her husband like the one he made to her she would rather rip her baby's smiling face from her nipple and bash its brains out than break that promise. At which point, he's right back in the game.
So her words do definitely move him back to murder.
But is she guilty of murder? I would argue no. Macbeth has a mind. He simply doesn't use it. He follows Lady Macbeth's direction, certainly, but has he no choice? I would argue that he does, that as powerful as her emotional argument is, that Macbeth has always had free will and has made his decision (albeit a bad one). The fact that Macbeth seems to perpetually follow bad advice does not make him any less responsible for his actions.
Of course, others would argue that Lady Macbeth's manipulation is so complete that he has no choice, that the threat of spending a night, nay, a lifetime on the couch is simply too much for a man to bear.