To be fair, the only reason Ma Chao sided with Liu Bei was because he was left outcast when the glorious and righteous governer Cao Cao had his dad assasinated, and on his own he didn't have anywhere near the resources to attempt to take revenge. By all accounts Liu Bei never actually trusted that Ma Chao was on his side so gave him a high rank to appease him and then kept him the smurf away from him because he was afraid of Ma Chao. /RotK nerd