Their hearts turned back to Egypt.
They were free, free at last. Generations of their mothers and fathers had labored as slaves and died in that foreign land, but they were breathing the air of freedom. They had ample food. They had pure water to drink. They were led by a man whom God had called, a man unique in history. They had a promise and a hope before them. By all rights they ought to have been looking forward.
They did not look, they did not move forward. They willfully traded their glorious inheritance in heaven for what they themselves had made, and they frolicked in the mire of their making. "Hey, we haven't a clue as to what happened to Moses, he may never show his face here again, so let's party down here in this godforsaken wilderness. In fact, if there's a caravan headed back to Egypt, let's hop on board."
Stephen knew that the likes of that crew, the type who didn't know freedom when it was openly offered to them, didn't become extinct in the Sinai desert. They were back, in fact they were in charge of things again, at least to an extent. They had an interest, an ungodly interest, in maintaining a relationship with idolatrous authority, so long as it facilitated their power. Change to their system, and especially a change that gave the people ideas about freedom, was dangerous, and must be quelled at all cost.
Stephen was building his case. He was making plain just what those authoritiesere doing. They were squirming in place. They were getting steamed. They were not going to abide this open provocation.
Stephen, like his Lord, was in danger even as he recited the warrant. But he wasn't going to stop telling the truth.