Darkness in my brain...

So, my confusion with this story continues. I find that the narrators opinion of Kurtz is very transient and changes every ten pages or so. I have to ask, up to the point of the story that I am, as Marlow chases a sickly and crawling Kurtz through the grass with fires blazing and pilgrims gunfire blazing around them, why is Marlow now trying to kill Kurtz? Oh yeah why, is there fires and guns blazing?

As I understand Marlow was sent to retrieve Kurtz from the jungle. I do not remember it being dead or alive. That method seems now to be the case. Previously I was under the impression that Marlow admired Kurtz for his success in the ivory trade. Now things seem obviously to have changed. Can someone tell me why Marlow's view of Kurtz has changed? Is it because Marlow knows that Kurtz will not go peacefully and therefore the only method for his extradition is through killing him?

As I read I am not dissecting Conrad's depiction of the duality of man, I am only trying to sort through the multiplicity of changing character outlooks, attitudes and motivations. I guess the multiple personalities of Marlow can be seen as duality but it is more like a split personality.

What is going on!?

Post Script: Oh wait, they are friends again now. Marlow is admiring Kurtz's greatness as he dies on the boat.