Explain why Hitler turned against Rohm and the SA in 1934.

Rohm and Hitler had completely different ideas as to the purpose and function of the SA within the new Nazi state. Rohm was intensely suspicious of the hierarchy of the German Army. He saw them as reactionaries who'd simply jumped on the Nazi bandwagon but weren't really committed to National Socialism. He wanted the SA to replace the German Army and for it to become an ideological armed force, something along the lines of the Red Army in the Soviet Union.
Hitler did not envisage the SA as performing this role. He knew that he needed the support of the German Army to consolidate his power and to fulfill his foreign policy objectives. The Army was the only institution left in Germany that could possibly have toppled Hitler from power, so it was crucial for him to try and keep the generals on his side.
The Army generals, in common with many Germans, looked upon the SA as a bunch of rowdy thugs undermining the stability and order that the Nazis had promised. To Hitler, they were becoming a growing embarrassment. The SA had proved useful in helping the Nazis achieve their fearsome reputation and in taking the fight to their political opponents. But now they were almost obsolete. And Hitler genuinely feared that Rohm and other leaders of the SA were about to stage a coup. Rohm was becoming a dangerous liability, and so it was decided that he had to be taken out.