The word "punitive" means an act that is meant to punish, reprimand, or inflicted with some pain or revenge, etc. This means that it is more connected to the act of being punished or given harsh dealings.
In the given passage, the word "punitive" is used in such a way that it is suggestive of the punishment the British inflicted or want to inflict on the African nation. Britain organized an expedition with the intention of inflicting punishment as revenge for the killing of the British delegates in what is termed as the "Benin Disaster".
