Any blocks that client produced would not match the expectations of the other clients. The difficulty being different would mean a successful hash that solves the block may not be seen as solving it by the other clients. They would reject it and your cheating client would be on its own fork.

If you could convince others to run your client then your network would get bigger. If it got >50% of the entire network then your blockchain would effectively be the main chain.

It would also have to maintain the >50% hashing power for more than 120 blocks. And even then it's not guaranteed as smaller pools could get lucky and solve more blocks than the larger rouge miner.