The only reason you wouldn't sign the NPT, is because you intend to sell nuclear weapons to terrorists.Even though Israel could or could not have nuclear weapons, it is not a member of the non proliferation treaty.
Which means Israel isn't violating any of its commitments. Plus, Israel isn't a phariah state run by radical extremists and the world biggest sponsor of Islamic terrorism like Iran is.
Treaty on the Non-Proliferation of Nuclear Weapons - Wikipedia the free encyclopedia
"member states have never joined the NPT: India, Israel, Pakistan and South Sudan. The treaty recognizes five states as nuclear-weapon states: the United States, Russia, the United Kingdom, France, and China (also the five permanent members of the United Nations Security Council)."