In "Heart of Darkness", why does Marlow believe the natives will not attack?

A couple of reasons: first, there is a thick fog in which the natives could get lost. Secondly, Marlow does not see any canoes that they could use to pursue their boat. What makes the thought of the natives attacking most "inconceivable" to Marlow, however, is "the nature of their noise" which does not have "the fierce character boding of immediate hostile attention." Rather, the sounds of the natives gives Marlow "an irresistable impression of sorrow" instead.