Some people are gullible, or trusting. This doesn't mean they don't have other gifts, or most of all good intentions. Applying an evolutionary pressure against people who can't fend off knife attackers is ultimately useless for human wellbeing
The real issue in the world isn't people not being smart enough to fend off attacks, but greed and ego
And besides, what is the root of such a 'idiot' person's deficiencies in the first place? Is it genetics? Or is it education, upbringing, early life traumas that stunt development?
Besides, the whole point of having fit genes in the first place is to bring about human happiness. If we use this to make people miserable it then shows that there is no substance to our perspective
> O CHILDREN OF MEN! Know ye not why We created you all from the same dust? That no one should exalt himself over the other. Ponder at all times in your hearts how ye were created. Since We have created you all from one same substance it is incumbent on you to be even as one soul, to walk with the same feet, eat with the same mouth and dwell in the same land, that from your inmost being, by your deeds and actions, the signs of oneness and the essence of detachment may be made manifest. Such is My counsel to you, O concourse of light! Heed ye this counsel that ye may obtain the fruit of holiness from the tree of wondrous glory.
> ~ Bahá’u’lláh