Because she'd be the only one still remotely fucking with him at this point based on the purity of his product. He could front like he's back in the game and sprinkle something in her tea, sure. I guess it would achieve, what exactly? Killing off everyone with any connection to Heisenberg whatsoever? And again: Why not just shoot her? Plus, it's not like she's just kicking her louboutins up at the aryan compound every day, so it seems like a hit of that nature would require some recon work that I can't see Walt having the opportunity to do.

But it's an interesting thought