Maybe he will use the poison on Lydia?
Unfortunately I don't see how someone as paranoid as she is would agree to meet with him anywhere.

This show really does detail how good and evil are all mixed up. Walt's definitely evil but he's not the worst of the bunch. But he can't stop lying to himself.

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.