Ernst Rohm was the head of the SA, the brownshirts. It was the single larges part of the Nazi party. Why they always make a big deal about him being killed because he was gay, it had more to do with Himmler and the German Army want him dead. And that he had power to oppose Hitler.
Röhm had pushed back against some Nazi policies, and with Brownshirt backup, he could have stood his ground. That sealed it for him.
He also hated the German Army's officer corps and wanted a "People's Army". Combined with the fact that Himmler wanted the SS to have more power, he was doomed.