Question: Why does Boris not just chop Frankie's entire arm off the case instead of wrapping it in newspaper and taking the bloody stump with him?
Answer: It saves time, and it's a little twisted. Would you chase after a guy who you just saw cut off a dead guy's hand to get a briefcase, then just decided to take the hand with him?
Answer: Basically, yes. He says it once when he discovers Tommy is carrying a handgun, which are not at all common in England, where they are outlawed. More the kind of thing a Brit would expect from a war-hungry Nazi. Later on he refers to it again just to mess with Tommy.
Rooster of Doom