Since we are able to calculate the aftermath and result with Lincoln I think Abe had a much better case to stand by. Although his first priority and intention was to preserve the union, ending slavery came with that. Which is a big deal. It’s very likely that slavery would’ve persisted for significantly longer had that event not happened. And that degree of slavery is one of the worst things any government can enforce/protect.
Putin’s ethical argument is that some Ukrainians feel more Russian? And that Russia used to exercise control over Ukraine so it should again? Weak.
Also to be fair I have to include Russia’s argument that the West is using Ukraine for strategic geopolitical purposes that may work against Russia. But Russia is an imperialist totalitarian government in its own right, so I don’t have much sympathy for one imperialist nation initiating war (one of governments worst atrocities) against a somewhat independent nation that has some connections to another imperialist nation.