How does Quatermain frighten people?
Throughout the novel, Quatermain relies strongly on superstition and threats to instill fear.
Quatermain represents himself as an honorable and practical person, but also a bit of a coward in his own words, and preferring to avoid fights if possible. Thus it comes as no surprise that he's more manipulative than Good or Curtis, and when manipulation is necessary he uses his extensive knowledge and superior technology to appear more powerful than he is.
He first does this at the home of a farmer to whom they entrust some of their gear before crossing the desert. Quatermain is doubtful of the man's honesty, and threatens various and thorough curses on the man, such as the death of all his cattle and even haunting by Quatermain's ghost should he die. Quatermain makes more extensive demonstrations with the Kukuanas, calling his gun a "magic tube" that kills by sound alone, and culminating with his darkening of the sun (an eclipse).
Being well aware of the limits of his abilities and the retribution that he will suffer if his deception is revealed, he tends to accompany each superstition with an empirical show of force, such as using the guns to kill an animal, and letting the "wonder" of the display speak for itself.