The original 歹 was supposed to be pronounced as ngat, or hiat, and said to be read the same way as 遏. The phainn or dai reading happened very late and old dictionaries specifically pointed out it’s wrong.
【俗書正誤】歹,音遏。【長箋】今誤讀等在切,爲好字之反。
Some say the wrong reading came from Mongolian influence, and was borrowed from the Tibetan script ཏ, which was transcribed as 𣦶, and not 歹.
So if you really want to get down to it, both mainstream pronunciations for 歹 in Mandarin and Taigi are wrong. Not only that, 歹 never had the meaning of bad prior to the Mongols. It originally mean bones with mean removed.
Taigi’s pháinn has 7 different usages, and probably came from 7 different characters, 否, 㤄, 敗, 敝, 圮, 𢻹, 㽹.