Funny enough, he was actually executing many of his supporters. The SA's/brownshirts helped Hitler rise to power; they were the Nazi party's own paramilitary wing.
However, they were also a political liability. As mostly working-class people (often left jobless in the lurch of the Great Depression), they wanted Hitler to follow thru on his promises of redistributing wealth. This brought them into the conflict with middle/upper classes and the army (which had deep root in the Prussian aristocracy). Taking out key SA leaders gained him massive approval from the army (which, as chancellor, he had not previously been able to control). Shortly after, he justified his action against "treasonous ringleaders" and passed retroactive legislation authorizing the killings.