(Define a good kill)
A job completed (to the letter of the contract or mission if that should be the case) without any harm to yourself or any innocents nearby.
I also think a good kill is one you can look back on without regret. Where you feel you are justified in your actions and maybe can even brag to yourself about. Knowing that justice or morals are served by the death is a wonderful feeling to have. Feeling you are speeding up the karma of the wicked is truly a wonderful thing.