What does charlotte imply when she says that zachariah looks like “the very imp of death in search of souls”?

Charlotte is both implying that Zachariah is ugly, in that he looks like an imp, which is a type of demon, but also that he looks malevolent or dangerous. He looks like Death on the quest for the souls of the dead. Like he's going to take them to some terrible afterlife. That means he probably looks pretty scary.