What the deadlist martial art?

Answer 1

There is no way to quantify degrees of being "deadly" ("deadlier"; "deadliest"). You are either dead, or you are not. Something is either deadly, or it is not deadly. All systems of Martial Art contain skills that are capable of killing someone, therefore they are all potentially "deadly," but none are inherently more deadly than others, nor is there a "deadliest" Martial Art. No genuine Martial Art promotes killing more than any other, and the ending of a life is done only when necessary to protect or save innocent lives - lest we become brutes, barbarians, gangsters or common criminals rather than Martial Artists.

Whether or not the skills are used for actions that result in death, and how often this occurs, is entirely up to the practitioner when applying their potentially deadly skills.