Why can't a transaction be redirected after it is signed? Scenario: 

Step 1: User-1 initiates the TXN-1 to pay to User-2 and the TXN is not yet confirmed with the ongoing block POW. 

Step 2: What prevents fraudster (anyone a miner or someone listening/participating in the network), to double spend or divert the payment to User-3 with a new TXN-2 by copying the data from pending TXN-1 including the User-1's signature? 

Step 3: if the miners reject both TXN-1 and TXN-2 as a suspicious or double spend activity in the current block POW, what prevents the same fraudster to issue a new TXN-3 by copying the TXN-2 with the next block POW? 

Thanks in advance.
The signature corresponds to the transaction it signs. You can't use the same sig to send the money to another address, because that would require a different transaction for which the signature would not resolve to valid.