Putting severed heads everywhere just screams "Dark Ages." Maybe Kurtz could try getting pastel lamp shades and laying down some tatami mats?
We get the sense Kurtz thinks that having severed heads and crucified bodies everywhere makes a statement: brutality (and scaring the daylights out of your enemies) is the only way to win the war. He learned that "lesson" when his enemies hacked off the inoculated arms of the children in a village. It was a kind of sickening revelation to him. The trauma of seeing that was a turning point in Kurtz's descent into madness, although he views it as a moment of great clarity for him.
Helpful hint from Shmoop: if you run into someone with severed heads all over their lawn and it's not Halloween, get out ASAP.