As far as centralized exchanges go, anything that prevents them from sticking their noses where they don't belong (such as CoinJoins) automatically triggers their "suspicious activity" algorithms and demands KYC.
You won't be in trouble legally, since there will be no way to prove if the funds you deposited were linked to darknet markets, and even if they were, if it was you that was using them on the darknet. What will happen is that your BitMex account will be locked and your coins will essentially be confiscated. If they like your KYC, they might unlock your account. If they don't like it, they will keep your coins and there will be next to nothing you can do about it.
Imagine the outrage when Binance finally enforces AML/KYC.
Better that they start being honest and saying that they will demand KYC from everyone, rather than falsely advertising their "2 BTC a day unverified accounts", and still slapping KYC on anyone who dares to do something shady such as log in from two different IP addresses.