These was nothing that could stop Putin from invading because he wanted to expand Russia's borders. That pretty clear to everyone.
Exactly, it wasn't just about joining NATO which Ukraine has been trying to many years. You look at what excuses Putin has been trying to do recently to justify invading Ukraine.
1. Being NATO at Russia's borders even though there are other NATO countries that borders Russia.
2. Protecting DNR/LNR backed by Russia
3. Recognize them as countries and claim they were under attack by Ukraine and invade few days later.
4. Claim Ukraine has biological weapons.
5. Claim Ukraine has nuclear weapons.
6. Claim to protecting Russian speaking people from Kharkiv to Odessa and other cities, not sure how far Putin was going, probably all the way to western Ukraine (ironically many Ukrainian soldiers can speak Russian so they can easily interrogate the Russian prisoners).
7. Ukrainian government are NAZIs as well as the military.
8. Ukraine has a military, enough to justify invasion since they want it demilitarize.
So many excuses, even if Ukraine was neutral, that's still enough to justify getting invaded since that's not the same as being Pro-Russian.