Good writeup on Roehm.
Roehm had become a serious opponent of Hitler's in the months before his arrest and execution. For one, his brazen homosexuality was rightfully seen as a total embarassment to the Party. There were other homos in the NSDAP, but no one as flagrant about it as Roehm was. Roehm was too much of a radical revolutionary even for most Nazis. He wanted to sweep away the old Prussian aristocracy and make the SA the new German army. This made many people uncomfortable, especially the wealthy German industrialists who Hitler depended on to finance his legal revolution.
Without Roehm's help in the early years Hitler would probably have never come to power. After Hitler was appointed Chancellor, Roehm and Hitler were on a collision course. Hitler made many compromises which Roehm saw as "selling out" the old warriors, the Party faithful who had stood with Hitler in the early years. Roehm would make comments behind Hitler's back about Hitler being a "traitor" and such. Can you imagine?
Roehm was a total megalomaniac and really only concerned with his own advancement, and was locked in a power struggle with Himmler and the SS. It was Himmler who convinced Hitler to have Roehm arrested and executed.
Hitler vacillated many times before finally giving in to Himmler's demands that Roehm be arrested and put to death. He told Hitler his old friend Roehm was planning a coup against him. Despite all of Roehm's boasting, there was no real coup. Had he not been a brazen homo, he would have had to have been eliminated by Himmler eventually for being too much of a loose cannon. The SA was hard enough to control as it was, but with Roehm leading them they were a serious threat to Hitler's new government.