Can an attacker use a modified Bitcoin node to pass fake transactions into the mempool? 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?
[The miner] edits 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?


Yes and no. 

Yes, it is entirely possible to modify the code to include whatever sort of transaction validation rules you want. 

No, you cannot use this to trick the rest of the network into accepting your invalid transactions. Your node would be banned by other nodes in the network, since it is not following the rules of the network. The network is formed by the connections of thousands of individual nodes, that all follow the same rules. If a node breaks those rules, then the rest of the network will simply ignore them. 

By changing the validation/consensus rules, you would effectively be creating a new altcoin network.