Lines 349-359 why might this part of kings speech be considered controversial

The excerpt that you are mentioning, between lines 349 and 359 belongs to  Dr. Martin Luther King Jr´s speech "Beyond Vietnam", which he delivered to the world in 1967. This entire speech came to people in America almost like a bucketfull of cold water at the height of heat due to the Vietnam War, mostly because it portrayed the rising feelings of rejection towards the war on the side of the American people, but also because it brought people to realize why America had been wrong in pursuing this war.

Essentially, while Dr. King expounds on the need for America, and the world at large, to oppose Communism, and rather fight to expand, and improve, democracy, Dr. King also exposes Americans to a truly harsh reality. The first, that America has forgotten what it is like to fight for what its right: for justice and liberty, for love, rather than a simple desire to defeat the Communist threat. But between lines 349 and 359, particularly, Dr. King starts calling America out on its forgetfullness of what is right, of doing things for the right reasons and expounds that, due to success, America has forgotten a lot of the ideals that made her such an important nation, and a front against Communism. In fact, it seems that Dr. King almost reminds people that while America grew comfortable in her own good times, and initiated the Vietnam War simply to repel a system that questioned their desire for money, comfort, and welfare, it seemed as if Communism had turned out to be the system that fought for the welfare of the poor, the oppressed. He states that in his speech, almost like a slap, given that up to that point, the whole reason for America fighting in Vietnam had been to repel, and protect, other nations from the evil spread of Communism. Dr. King shows America another reality, and almost places Communism as an example, in those lines only, of what America should be doing.

This is why these particular lines might have caused a lot of controversy, even amongst Dr. King´s own followers.