What is the meaning of this expression (落了下乘)?
乘，梵语 Yāna，旧译为“衍”。有一乘、二乘、三乘、四乘、五乘之别。

以五乘来论，五乘也有多种说法；其中较常见的是指：人乘、天乘、声闻乘、缘觉乘、菩萨乘。这是由下品到上品的顺序。

文中先提到“虽为杀人事， 亦是菩提心”。

“菩提心”是度众生之心、成佛之心，属“菩萨乘”，也就是最上品之心。

“杀人念”是“人乘”的念头，是五乘中最下品之心。

因为有了杀人的想法，就由最上品的“菩萨乘”降成最下品的“人乘”，所以才会说“落了下乘”，堕落成为下乘。



乘 [shèng | ㄕㄥˋ] (Sanskrit यानम Yāna) the old transliteration is 衍那, simplified as 衍.
It means (noun) a vehicle or (verb) to carry.

There are five kinds of 乘: 一乘, 二乘, 三乘, 四乘 and 五乘.
It is better to answer the question in terms of 五乘.

五乘 also has a few kinds.
The more common one is: human beings, gods, श्रावक śrāvaka (hearers), प्रत्येकबुद्ध pratyekabuddha (beings who enlightened through reasoning on the riddle of life), and बोधिसत्त्व bodhisattva (conscious beings of or for the great intelligence).
This is the order from the lower level to the higher one.

He first mentioned, "Although it is a killing, is still a bodhichitta."

"Bodhicitta" is the mind of helping the sentient beings and of becoming a Buddha.
It is the highest level.

"Killing" is the idea of human beings.
It is the lowest level.

Because of the idea of killing, the mind falls from the highest to the lowest level.
Therefore, he said 落了下乘 (to fall to a lower level).