Is it possible for someone to theoretically take over the bitcoin network and create their own Bitcoins if they own 100% of the hash power?
Anyone who mines a block can get the block reward. However, the miner CANNOT include the amount that is more than the block reward and transaction fees in their coinbase transaction. Reference node would simply reject the block as being invalid.
However, they can potentially refuse to confirm any transactions, the user can reverse their own transaction that was created after the block checkpoint. In a sense, they are controlling which transactions can be confirmed and which won't.