If the victim is truly helpless, then they can just kill them with no roll necessary. Common sense should always trump the rules. ([Basic Rules, p. 4][1])  

1. The DM describes the environment.   
2. The players describe what they want to do. 
3. The DM narrates the results of the adventurers’ actions.  


  [1]: https://media.wizards.com/2018/dnd/downloads/DnD_BasicRules_2018.pdf#page=4