CF is a roleplaying game. In CF, RP means playing a character and doing things as your character would do. PK is part of this system. So in short, you will PK someone because that is what your character would do in that situation. PK is part of roleplay. This is why it is a good thing that PK isn't encouraged with non-RP-related rewards. A skilled killer may be acknowledged by Immortals(if he plays the role well) or feared by mortals. Fame comes like real fame, it's not hard-coded.