Why??

Taylor was slowly and slowly drifting into the mindset of Barnes, killing his own man in cold blood was the wrong thing to do but by the end of the film he did it without hesitation, Chris at the beginning of the film would not have done that. Throughout the film he was teetering on the line between good and evil and by the end he was over the edge and had become the new Barnes. Barnes killed his own man in cold blood and justified it to himself (if he gets court martialed then the platoon is going to fall apart (which is probably would have) and the men are going to get killed), Taylor killed his own man in cold blood and also justified it to himself. Fortunately he was saved by being allowed to come home where he will hopefully be able to regain his humanity. To win a war you have to do immoral things, there are no half measures.

Barnes did not care about morality, but he was scared of being subjected to a higher judgement, as in a military court. He didn’t want to lose face and be stripped of his position. He wanted to die an honourable soldier’s death, and Taylor granted him that.

Barnes was maybe a good soldier before, but as Elias said, he still believed in the purpose of the war and that what he was doing was worth it. He could not see that he was fighting a lost cause. Barnes was given too much power in the platoon and eventually became megalomaniacal. Barnes needed to be put in his place to be an effective soldier, but by the time the film starts, anyone who got in his way, Barnes would bully into submission or remove from the equation. Quite simply, Barnes became a bad machine.