I don't think "Captain" refers to how many people you can kill. It is a rank used in military organisations, and a title used in civil organisations. It goes with the uniform that those organisations also require people to wear. It's part of the contractual relationship between the organisation and the individual given the title and requred to wear the uniform.
If you are flying a C150 or even a 747 in a non-commercial private capacity neither title nor uniform have much significance except to the individual, except perhaps in under-developed countries where officials are wowed by bars. And at fancy dress parties, of course.
Debating whether the person in charge of a C150, a 747 or a dinghy "ought" to be called "captain" rather misses the point IMHO.