1. If Russia could have pulled that one before, they would have. They've already broken every and any international laws, conventions, etc. that were in their way. The only reason they've not used nukes is most likely because China would instantly turn their back on them, and they can't afford that.
2. Ukraine is under threat of annihilation and they never got half the support they needed from their allies. Everything you point about "long term issue" is secondary to them because if they don't keep that technological advance on Russia, they won't exist to see that "long term issue" anyway. You want them to keep some reasonable limit? Write to whatever representative you have in your country and ask for a massive and immediate support to Ukraine. Had they done that on day1, the war would have been over for years and we wouldn't have seen that massive technological "progress" in military equipment.
3. Nukes were invented during a high intensity war, used, and then countries got reasonable enough to not use them again (so far…). Chemical weapons were invented then banned. Problematic weapons do get banned, though the rules are not always followed. That's why we need international laws, conventions, etc. These past years they were all but thrown away by a few countries without any consequences, therefore, don't expect any kind of holding back on terminator drones. They were to happen no matter what, and Ukraine just had the right conditions to be the first to get them. I would be very surprised if Israel was not already working on it.

And nukes have been rather good peacekeepers after the invention of those, so you cant really say that inventing new weapons doesn't make one safer.

The other person explained it rather well.

But in the face of annihilation, every possible advantage will be used. It's that simple. Long term and moral consequences are irrelevant, those can be dealt with later, but first one has to get to that later.

If you want to get really personal aka hammer in the point. You can imagine someone who already has broken into your neighbors home and done horrific things and is now breaking into your home and already starting to abuse/hurt/rape your loved ones. There is no police. Would you not use every single possible method to protect yourself and your loved ones? Or would you have philosophical discussion with the invader while they're raping your wife/daughter/sister or cutting off the balls with a box cutter of your husband/son/brother. Those aren't even made up examples, those are real examples with videos.