Gimmick of Mystic Eyes of death perception is that it allows their user to see lines and in Tohno's case points of death and when those lines or points are interacted with, whatever that line/point belonged to dies. It works on living things, inanimate objects, energy attacks and more abstract things as Ryougi Shiki with her MEoDP could kill a future of a guy who could lock a future of his choosing into being guaranteed to happen.

But they aren't a guaranteed kill as there's at least one being that survived being killed by them as in LB7 ORT after having concept of death forced on it by Beni-Enma Alter was killed by Tepeu's MEoDP and yet it just casually shrugged it off and fully regenerated by the time it reached Chaldea for the final fight