(1) does not belong to "contract fraud", and there is no criminal or administrative responsibility, just a civil dispute.
(2) Sublease without the consent of the landlord: Sublease contract belongs to the contract whose effectiveness is to be determined. If the landlord approves it afterwards, the contract is valid; If the landlord does not agree to sublease afterwards, the sublease contract is invalid, and the landlord can cancel the sublease contract and recover the house. You are responsible for the losses of the lessee you sublet, that's all.
2. "I want to ask, is my behavior illegal? Is it serious? Do I need to bear any legal responsibilities? " ：
(1) has some minor violations, but it only violates the contract law.
(2) The responsibility you should bear is: When the landlord refuses to ratify your sublease contract, the sublease contract is dissolved and the house is taken back by the landlord, you should bear the liability for breach of contract to the landlord and compensate the sublease for the losses.
3. "Is his request reasonable? Should I pay him? " ：
(1) It is reasonable for him to ask you for compensation, but the amount of compensation is unreasonable. You should terminate the contract in advance and bear the liability for breach of contract, that is, you should give the other party some time to find a new house and bear the other party's moving expenses reasonably.
You should compensate him because you have violated the lease contract between you and him. You broke the contract and should be liable for compensation for it, but not as much as he said.
4. "If I don't agree, I think I have two choices. First, return the deposit to him and let him move out. I don't care if he doesn't move. Let the landlord take over the house, and I won't pay the landlord the monthly rent any more. Second, if you can't move out again, you will be blackmailed for compensation. I simply don't even refund the deposit, just disappear. Anyway, if the landlord can't receive the rent, he will naturally accept the house. " ：
(1) Don't return his deposit easily before the negotiation, you may lose the weight of the negotiation.
(2) The second scheme: If you guarantee that the landlord can't find you within 2 years, you may be fine. If you still work in the local area, you can't escape, and you will always be found by the landlord and lessee, and you may bear more then.
5. "If I do this, is my behavior illegal and is it fraud?" It won't be a fraud case, but you may have to bear the corresponding civil liability.
6. "I did this, and the other party went to report the case. Will you file a case against me? I am not a native of Shanghai and left my ID number and name for the other party. " ：
(1) This is a civil dispute, and the public security should not take charge of it and will not file a case.
(2) But you are a native of Shanghai and have a work unit. It is not difficult for landlords and lessees to find you. If it is discovered, it may cost more (including legal fees).
7. Recommendations:
(1) If you can negotiate with the other party, you can compensate him for some moving expenses within the range you can afford, and let him move out as soon as possible without letting the landlord know. Or you can find him a new place and let him move out.
(2) If negotiation with him fails, directly explain to the landlord to see if the landlord can ratify the sublease contract, and ask him to pay the rent directly or let the landlord take over the house.
(3) You can also tell him that you have no right to sublet. If he doesn't move out as soon as possible after getting proper compensation from you, the landlord will take the house. If you disappear again, he may not get it at all and move.