Because in his mind, he doesn't directly benefit from the act in the former case, here he stands to gain the most.
There's also the fact that the former situation has a sense of "inevitability", the Guandong coalition would have robbed them if he didn't, here it is Zhang Fei deliberately doing these dishonourable acts.
I recommend joining the RoT discord server, we are more active there, I've only clicked on this thread at a glance.