For an attacker to try to insert fraudulent transactions into the Bitcoin ledger, he would need to have a majority of the processing power behind the network to accept his fraud. Honest nodes that are part of the network would have no incentive to do so, because it would undermine the integrity of Bitcoin and devalue the rewards they are receiving, wasting the electricity and resources they have expended on it. So an attacker's only hope would be to mobilize a quantity of processing power that constitutes more than 50% of the network to verify his fraud and build on it as if it were valid.
