You are blaming Communist Chinese aggression in Korea on MacArthur? You cannot be serious. The US was in no position to invade China, regarless of being on the border. The Chiese knew that. They wanted Korea as a buffer against Japan and the US in NE Asia.
I believe they would prefer a buffer, but I do NOT agree that they invaded just because the UN took the North. We had nukes and your buddy Big Mac was demanding we use them on China. He was demanding, long before China intervened, for us to bomb China, to invade China. And he kept demanding it publicly every chance he got including after being ORDERED to stop.
The Soviets supported North Korea, not the Chinese. The North Koreans were trained, equipped and supplied by the Soviets and most likely invaded at the behest of the Soviets.
The Chinese were ill equipped and were still securing control of their country after driving the Nationalists out. Now if you want to curse Truman, THAT is a valid complaint, he and Marshal are directly to blame for the loss of the Nationalists.
The Chinese did not even have enough rifles for their armies, nor an adequate supply of heavy weapons or ammunition. As forward troops died, follow on troops picked up the fallens weapons.
I suspect that the Chinese would have been willing to wait and see about Korea if Mac had not kept running his mouth. They invaded at the prodding of the Soviets. The Soviets could not intervene directly as Truman had already threatened them with nuclear attacks if they did. Further Soviet armies in Korea against western armies would have lead to WW3 in Europe. China did not have that problem, while huge they had no strategic capabilities and could only effect local conditions.
One could argue also that the UN invasion of North Korea was against the authorization to save the South. In fact the only reason the UN was involved at all was the Soviets were protesting the UN and were absent during the time the measure was drafted and the Security Council passed it. The Soviets had a veto power.