Ukraine wasn't going to be accepted into NATO anyway. They asked. Weren't accepted. Ukraine knows this, Russia knows this, NATO countries know this. It's just Russian excuse for propaganda reasons.
The real reason is that Russians consider Ukrainians "the same nation" and if Ukrainians entered EU or some sort of association with it, and got richer than Russians like Poles, Slovaks, Czechs, Balts etc. - it would be very bad for Putin approval in Russia. Russians would start to think "it could be us", "it could work without the tyranny".
So Putin can't let westernized, wealthy Ukraine to exist. Same reason he helped Lukashenko to stop revolution in Belarus recently.
The rest ("history" lectures, "NATO is encroaching", "Ukrainians are nazis and bombard Russian speakers in Ukraine") - it's just bullshit excuses. Don'r repeat them, please. You are helping an aggressive nationalist dictatorship murder people by accepting their excuses.