觀無量壽佛經疏 2.96

2.96

言修十善業者，此明十惡之中殺業最惡，故列之在初。十善之中長命最善，故以之相對也。已下九惡九善者，至下九品中，次應廣述。此明世善，又名慈下行也。

As for "cultivating the ten wholesome actions": this clarifies that among the ten unwholesome actions, the karma of killing is the most evil, and so it is listed first. Among the ten wholesome actions, long life is the most wholesome, and so it corresponds to it. The remaining nine unwholesome and nine wholesome actions should be explained extensively below under the nine grades. This clarifies worldly good, and is also called the practice of being compassionate toward inferiors.