I just saw some video explanations and got to know that a miner with 51% of hashing power can build blockchain faster than any other miner which is pretty obvious.
Also, he can reverse his previous transaction by creating a private blockchain and publishing it later. My question is why he cannot do the same while spending someone else's bitcoin, as he can still build on the previous block containing the invalid transaction, As his power is more than any other miner he will manage to build the longer chain even if other miners refuse to build over the invalid block.

Please, someone, take the pain of explaining this to me.