A continuation of the previous post about Legato having his own interests and goals in the manga and not just following Knives' wishes.
Here I'd like to say something about the last battle between Legato and Vash, because it wasn't part of Knives' plans, it was purely Legato's decision and it even conflicted with Knives' interests. When Legato joins Knives in the final battle, at first he's just holding Vash back. This is what Knives wants - for Vash to stay out of his way while he fights the Earthlings. Knives was probably counting on Vash and Legato to stay alive, and he'll decide what to do with them later. But Legato has completely different plans: he wants to compete with Vash to prove to himself and Knives that he is better than Vash. How does Knives feels about it doesn't really count for much.
Ironically, Knives was clearly glad that Legato joined him in this battle, his expression and demeanor had changed, his self-confidence had clearly increased, and he was clearly counting on Legato. So in a way, he had already recognized him. But as I said, Legato needed to prove to himself that he was stronger than Plant, and specifically Vash, and since he wasn't, there was no reason for him to continue to exist.
If it was important to him to help Knives with his plans, he would have done the same thing he did when he first met Vash: threatened to destroy every person in the area in two seconds if Vash doesn't sit still while Knives deals with humans, and considering Legato said his threads are everywhere, Vash would have to risk the entire surviving population of No Man's Land.
So here's the point I'm trying to make, Legato is loyal to Knives, but that loyalty isn't slavish obedience. Legato loves Knives, but in the manga, that love is rather possessive. This is especially noticeable when comparing his reasons for dying by Vash's hand in the old anime and in the manga. In the anime, he recognizes that as a human he is an inferior form of existence to the Plants
so Vash should be with Knives, and he forces Vash to go against his ideals because those ideals are harmful to Vash. That's exactly what Knives thinks. In the manga, however, Legato believes that he is everything that Knives needs, and Vash is the obstacle that has come between them.
Knives' doubts that Legato, a human, is capable of defeating Vash, a Plant, didn't so much make Legato "realize his place" as provoke his desire to defeat Vash while also limiting his own powers and not resorting to any hostage tricks. Therefore, Legato perceived Vash's refusal to kill him as a mockery, and forcing Vash to shoot him was a punishment for this "mockery".
Yeah, and despite the fact that for Legato, Vash was the obstacle between him and Knives, Legato liked the rivalry with him.
I think ideally Legato would like to not just get Knives for himself, but "win" him.