How can't a malicious attacker generate bitcoins on his private network?
The blockchain will only accept blocks which are based on previously accepted blocks and have a hash value that satisfies the current difficulty requirements. A malicious attacker is in the same position as any other miner, so any block that he mines must satisfy the same conditions regarding validity. In fact, the bitcoin protocol is designed to be trustless, which means that even miners with malicious intentions are not able to destroy its integrity unless they manage to control a majority of the mining power.