About what does Marlow...?

There are many rumors about Kurtz as they float down the river. Kurtz is something of an anomaly. To some white imperialists he is crazy and to others he is a traitor to his race. So, naturally there are plenty of lies spread about him. Marlow has no connection with Kurtz yet but explains how much he detests lies,

"You know I hate, detest, and can't bear a lie, not because I am straighter than the rest of us, but simply because it appalls me. There is a taint of death, a flavour of mortality in lies — which is exactly what I hate and detest in the world — what I want to forget. It makes me miserable and sick, like biting something rotten would do.... "

At this point Marlow isn't so much defending Kurtz more than condemning the fickle lies that are so abundant from his own people.