For their refusal to accept his request, Ukraine’s President said NATO is weak and disunited. Zelensky warned NATO that any Ukrainian that die from now on, died because they refused to assist his nation.
NATO is not weak. They are just cautious. Putin has lost his mind. It would be disastrous to be mad at an angry man. A proverb in my tribe states that “If a mad man takes your towel while you are bathing and you pursue him naked, people will assume that both of you are mad. If NATO is not careful, World War III would begin.
Yes. Big wars tend to start as small wars that get out of hand. It's a difficult balancing act for the west, they need to help Ukraine as much as possible without this being too overt. The two options for this ending well are probably either a) give Putin a way out where he can retain some dignity and maybe somehow be able to frame it domestically as a victory, or b) destabilise Putin's support sufficiently for them to decide to "remove" him.
Sending NATO/US forces into Ukraine will not end well. But it's certainly understandable that Zelensky wants whatever help he can get.