When I encounter these two delusional cultists as a Neutral Evil characte I have reason to believe that they are dangerous to me. I tell them to go and avenge the death of their brother by killing the owlbear. My intention is not to go on a Merry owlbear hunt and put my life at risk for these dangerous maniacs. I simply want to manipulate them through their religious zeal to get them killed in a futile owlbear hunt. Why is this not an optional outcome for this encounter?

You literally can... just tell them to go fight it and you can stay back and watch as the owl bear rips them apart.