梵網經菩薩戒本疏 4.66

4.66

初、制意者，菩薩理應捨自身肉以濟物命，何容反食眾生之肉？違害之甚，故須制也。

First, the intention of enacting [the precept]: bodhisattvas in principle ought to give up their own flesh to rescue the lives of creatures; how can they tolerate conversely eating the flesh of sentient beings? It is an extreme violation and injury; hence it had to be prohibited.