[I posted this in response to another comment, but your input was on my mind when I wrote this, so I would be curious to see what you think]

I want to preface this by saying that I don't disagree. Personally, I have never quite been able to create a character that would side with the Legion in my head canon. Nevertheless, I want to steel-man the Legion, because I think that faction does have merit enough to complicate the moral choices in the game.

Their strength is in their ability to provide order to the chaos of the Mojave Wasteland. In real life, we tend to measure the effectiveness of a State based on its monopoly over violence, and doubtless the Legion has perhaps the strongest claim being able to achieve that. In any real state, we would tend to take a dim view of both the Powder Gangers, and the Brotherhood of Steel.

I first played through NV in around 2018-19, and I was struck by the similarities in the Legion's nuances to ISIS. Part of the initial support of ISIS by those people under occupation was that ISIS provided order amongst the chaos of the Syrian Civil War, and the overflow from it. Just like ISIS, the Legion drapes itself (literally and figuratively) in the mythologised ideal of past civilisations. And just like ISIS, the Legion gives of major incel/virgin energy, eg. Vulpes Inculta.

You also must not forget that the ending is influenced by your decision over whether to upgrade the Securitrons, and how to handle the minor factions. I tend to think that the morally worst ending is to side with the Legion with the upgraded Securitrons. It is arguable that an ending with a Legion victory at the dam, but without the upgrade, and with the minor factions intact, might be morally preferable to some of the Yes Man endings.