This is correct; the "no kill" rule is something you aim for, it doesn't mean you necessarily achieve it. In Mahayana Buddhism there's a story about the Buddha's prior lives, where he decided to kill someone out of compassion: the victim wanted to murder a lot of people, but the future-Buddha killed him to avoid his suffering [1]. Likewise, the Buddhist precepts are training rules, not necessarily commandments that will take you to hell if you break them.
[1] http://venyifa.blogspot.com/2008/09/story-of-compassionate-s...