All things considered, the West really cocked up when we made Ukraine agree to that deal.
Not really. The US cocked up when it elected Clinton instead of re-electing Bush.
The eight years of Clinton was the most critical time period for moving Russia from enemy to ally. Post Soviet collapse the US should have thrown its full might into rebuilding Russian society and fully integrating it into our global order. Instead we elected the domestic President and Clinton had like a months worth of foreign policy over his eight years in office so we just let Russia do whatever and set the stage for the current mess.
Bush's whole second term was basically supposed to be deciding and laying the ground work for US grand strategy in a post Soviet world.
Instead the US has, realistically speaking, had basically zero foreign policy for the past thirty years. Or to be more precise, outside of the War on Terror, US foreign policy has continued basically on inertia with no coherent strategy or plan.
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As for chemical weapons; Ukraine would be fools to use them even if they had them. You do not fight a WMD war with Russia, especially not as a minor power that Russia has already invaded.
Give Putin the excuse and he will hit Kyiv with Sarin (VX is too persistent).
It's one of the big problems with seeing an end state for this war that is good for Ukraine. Ultimately, Ukraine can't win without going after the infrastructure and logistics in Russia proper. And to deal with those targets, Ukraine would need to send in substantial forces (not just a handful of drones or missile strikes). Violate Russian territorial integrity and under basically every nuclear powers ROE, Russia would be justified in going nuclear. Turing every Ukrainian city into a radioactive crater is an acceptable end state for this war for Russia.
And no, none of the other world powers would retaliate in kind. The US, France, and UK are not going to risk WW3 over the elimination of Ukraine. Of course, Russia would have their exports even further curtailed as Russian shipping is stopped from transiting the Med or Baltic seas.