Don't kill him. Or her.
Instead, be a bit kindly to them.
First, assuming that such a thing as Buddhahood exists, a real Buddha, like a Socrates, wouldn't claim to be a Buddha. Therefore, anyone making such a claim is still attached to ego, or doesn't know yet all that he doesn't know.
Second, the actual Buddha (if a Siddhartha Gautama actually existed) didn't have Buddhahood. Karma/reincarnation, as I've said before, is as offensive as heaven/hell, especially the conservative Christian original sin version of that. Perhaps even more. And, of course, from my point of view, karma/reincarnation has no more reality than heaven/hell. So, anyone claiming Buddhahood on the dogma of the dhamma deserves kindness. (Unless he or she stridently persists too much.)
Third, the metaphysical, or psychic, or even "just" psychological cause-and-effect claims of karma simply aren't true, anyway. Even if we set metaphysics aside.
Regular readers here know that I reject the idea of free will "versus" reincarnation. Well, that's why karma isn't real, beyond the metaphysical issues.
Psychological causes and effects simply aren't black and white like karma presents them as being. Rather, we are all driven by a mix of free will and the psychological "shaping" of previous events in our lives. No two individuals will have the same mix of free will and determinism in their different reactions to exactly the same situation. No one individual will have the same mix in his or her different reactions to the same, or nearly the same, situation, occurring at two different times in their lives.
So, be kind to any "Buddha in the road."
They're sadly deluded.
Be kind and help them out.