There were rumours of a conspiracy to overthrow Hitler, and that the SA was involved in this. Hitler saw Rohm as a threat to his leadership.
The German army did not trust the SA, and Hitler needed the support of the (regular) army.
Rohm, the leader of the SA, was a left wing Socialist, and wanted the Nazi Party to change its economic policy along those lines. Hitler did not accept any form of criticism from his followers. He also did not want to offend the industrialists who were supporting the government financially.
The Nazi leadership was disgusted by the openly homosexual preferences shown by some of the SA leaders.
Many SA members expected to become well-paid government officials when the Nazis took power.
It was an opportunity for Hitler to get rid of many people he saw as enemies and did not trust.
The SS was jealous of Rohm's power and of his position over them.
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