1. This is ambiguous: "How did Marlow consider Kurtz and why?"
It'd be better to write, "What did Marlow think of Kurtz and why?"
3. comma after "fail"
5. Who are "they"?
7. Delete "accidents" and insert "aspects" (word choice error).
11. Delete "Kurtz's" and insert "his own"
12. Change "to have" to "to have had" (verb tense should be past perfect).