Because it's obvious to anybody with a functioning brain that Ukraine will lose this war, and the only thing that's being accomplished in the process is that hundreds of thousands people die, and millions more have their lives ruined. Prolonging the conflict will not change the outcome.

The whole angle to talk about justifying the war is just empty moralizing. The reality of geopolitics is that might makes right. This is what US led "rules based" order has established since the fall of USSR. NATO has invaded and destroyed numerous countries, and they were able to do that because nobody could stand up to them. There were no consequences for any western leaders, and no justifications are necessary. Now Russia is doing the same, the only reason westerners seem to be upset about it is because it's somebody else doing the same things the west has been doing.