After Tom witnesses Injun Joe kill Dr. Robinson and frame Muff Potter, he is full of guilt and fear, and Aunt Polly misinterprets his reaction as an illness, leading her to try various remedies. Here, Twain uses a metaphor to describe Aunt Polly’s poor healing abilities—comparing her to death itself:
She gathered together her quack periodicals and her quack medicines, and thus armed with death, went about on her pale horse, metaphorically speaking, with "hell following after." But she never suspected that she was not an agent of healing and the balm of Gilead in disguise, to the suffering neighbors.
Twain notes directly in his narration that he is using a metaphor and puts “hell following after” in quotes because it is a biblical allusion—Revelation 6:8 describes a personified Death as riding on a pale horse with hell following after him. Together, Twain’s use of metaphor and allusion are a tongue-in-cheek way for him to mock Christians using their own language.
What Twain is getting at here is the hypocritical ways in which religious adults chide children for their superstitions and fantasies but approach medicine and religion in a very similar way. Aunt Polly is using “quack medicines” but “never suspect[s] that she [is] not an agent of healing” because she believes the fantastical stories that the people selling the medicines told her. “Hell” does not actually follow after her, as the metaphor suggests, but Tom is certainly harmed by her attempts to heal his emotional agitation with unhelpful, bitter remedies.
