Peace the bosnian thinker. The context of 493 is believers fighting the munafekeen/Mushrikeen. The only reason /way of a believer to kill another believer deliberately is If a believer takes advantage of the war situation to kill another who they know is believer for their own personal motive /to gain something /jealousy/personal ......They may claim that he is a munafiq or accuse them of something ...etc In that case this believer has disobeyed and discarded GOD s instructions/command knowingly. Hence there will be no forgiveness. There is no other possibility,they are at war with the other side and are allowed to defend themselves.i.e killing "disbeliever" deliberately without a cause does not arise in that situation since the other ones are fighting them. Innocent "disbelievers will not be there fighting ! The believer who has committed such crime will know that the outcome is gehenna ,hence it is clarified in the verse. He has disbelieved after his belief and committed the crime knowingly and intentionally. Hope this clarifies the context for you, GOD bless you. Peace.