I'm afraid it may just come down to good old-fashioned stupidity. In their songs, the ICP often talk about cutting people's heads off, and I'm guessing these goofs just decided to take it to the next level. They really just wanted to rob people, but they threw this behavior in as some sort of stylistic choice.

Then again, they might be whacked out of their minds. I hope so. At least then there would at least be some sort of explanation for their behavior.