Why did Discovery's Crew treat
as an enemy after they found out that
he was from another universe? His goal was the overthrow of the Terran Empire.
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Up until Lorca's reveal, the crew believed him to be the Lorca of their Universe. A man with a known history in Star Fleet, with a known reputation.
When it is revealed who Lorca really is, the crew can no longer trust him. He's been lying to them for a long time; the crew can't be certain what was true and what wasn't.
So now they cannot trust his motives. They cannot trust how much of their personal history with him is true. They cannot trust how many of his orders were intended to further his plans.
Importantly, Mirror Lorca deliberately altered the spore drive to get to the Mirror universe. This bought the Discovery crew into danger and nearly killed Stamets.
Treating him as an enemy is a defensive move: everything he does will now be met with suspicion and scrutiny. Also, Lorca loses all his privileges and rights. His previous orders can be ignored.