Is it possible for a malicious attacker to download a bitcoin software code, modify that code and use that software for his own benefit? I know that any changes implemented in the bitcoin software must be accepted by the majority nodes in the bitcoin network. But, here, consider that the malicious attacker doesn't need to use that modified version of bitcoin software by other users.
Here is an example, Suppose B is a malicious miner. He wants to modify the verification conditions(20 rules) of transactions. Then, he edits the code of bitcoin software to set his own verification rules for transactions so that he can bypass all the invalid transactions from his accomplices to mempool. Is it possible?