I tried to correct the wrong sentences myself. Please, tell me if everything is ok.

1) Then, Marlow went into the broken-down hut and found an old book about seamanship with notes written in code.
2) He came to Africa with the intention of civilizating the natives.
He had a highly moral view of the European role in Africa.
3) He was looked upon by the natives because he came to Africa with the guns, and the natives had seen anything like them (or that?) before.
4) He failed his mission because he didn't have enough strength of character too face the solitude and wilderness of the jungle.
5)The first person they met was a Russian, who traded ivory. He looked like a harlequin in his higly colored clothes with patches in them.
6) His soul was struggling between his love of what he had discovered, and the hatred for what he had discovered.

2. civilizing

3. doesn't make sense at all. He was looked upon by the natives" doesn' say anything. Work on all the sentences here.
4. "to face"
5. highly....with patches on them.
6. discovered and his hatred for what he had discovered.

1. remove comma after "then"

3. he was looked down upon...
??

1) The sentence is grammatically correct. However, to improve clarity, you could replace "with notes written in code" with "which had notes written in code."

2) The sentence is incorrect. The word "civilizating" should be replaced with "civilizing." The corrected sentence should be: "He came to Africa with the intention of civilizing the natives. He had a highly moral view of the European role in Africa."
3) The sentence is grammatically incorrect. It should be: "He was looked upon by the natives because he came to Africa with guns, and the natives had never seen anything like them before."
4) The sentence is grammatically correct. However, to improve clarity, you could rephrase it as: "He failed his mission because he didn't have enough strength of character to face the solitude and wilderness of the jungle."
5) The sentence is grammatically correct. However, to improve clarity, you could rephrase it as: "The first person they met was a Russian who traded ivory. He looked like a harlequin in his highly colored clothes with patches."
6) The sentence is grammatically correct.