This phrase ("If you meet a Buddha on the road, kill him.") is an old misunderstanding of a teaching given by the great Tang Dynasty Chinese Zen master, Linji (d. 866).
Here's the full context of Linji's teaching:
"Followers of the Way [of Zen], if you want to get the kind of understanding that accords with the Dharma, never be misled by others."
"Whether you're facing inward or facing outward, whatever you meet up with, just kill it!"
"If you meet a buddha, kill the buddha. If you meet a patriarch, kill the patriarch. If you meet an arhat, kill the arhat. If you meet your parents, kill your parents. If you meet your kinfolk, kill your kinfolk."
"Then for the first time you will gain emancipation, will not be entangled with things, will pass freely anywhere you wish to go."
As the full context makes clear, Linji was not instructing Zen practitioners to go out and kill everyone they meet. Rather, he asked his followers not to "be entangled" with any concepts, including titles such as "buddha," "patriarch," or "parent."
Practitioners must "kill" these concepts whenever they appear in the mind.
When someone is not entangled in concepts, then they can encounter the world as it actually is, without the intermediation of ideas, opinions and beliefs. Linji calls this "emancipation." The Buddha called it "enlightenment."