It baffles me that anyone could be so morally empty as to believe this.

First and foremost, Ukraine applied to enter NATO before the war, and was turned down, because NATO didn't want to piss off Putin.

That fact alone makes that claim hot smelly garbage.

But imagine that Ukraine were in fact going to join NATO in future.

How does this justify mass murder and destruction?

It doesn't. Only a psychopath would believe in the idea of committing pre-emptive genocide.

Disgusting.