Thank you very much for your corrections. Here are some more sentences I'd like you to check.
1.Kurtz wanted to realize (to fulfil) his highly moral ideals but he failed. He wanted his ideals to come true but he failed because he was left alone to face the wilderness of the jungle. The jungle had gone into him and taken hold of him.
2.Actually, he even took part in the natives’ (?) midnight dances and in their terrible ceremonies. He also decorated the wooden post surrounding his house with human skulls.
3.The natives thought he was like a God on earth mainly because of his guns (can you say: mainly for his guns?).He became subject (a slave, a prey ?) to his own desires. His desires drove him mad.
4.He was dressed in highly (AND NOT very) coloured clothes and appeared like a harlequin ( correction: he looked like a harlequin). His clothes made him appear like a harlequin. The Russian was colorfully dressed. He wore an ironed and starched white shirt. (can you also say: he was dressed in…./ he had …. on ?)
5.Before dying Kurtz struggled against the darkness and the horror which his actions had caused on his soul. He struggled against the madness of his soul due to the terrible things he had experienced.
6.He eventually (?) or Eventually, he looked back at the terrible things he had committed and was horrified by the sight. Kurtz had the courage to look into the darkness of his soul.