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You can't get much clearer than Rambam, Hilchot Ma'achalot Assurot 4:6:

שאם מתה הרי היא נבלה ומה לי מתה מחמת עצמה או הכה בסייף והמיתה או שברה ארי והמיתה הא אינו מדבר אלא בשנטרפה ולא מתה:

for if it died, it is a nevelah. What difference does it make if it died naturally, was struck by a sword or died, or was battered by a lion and died? Thus [the term trefe] must refer to an instance when it was mortally wounded, but did not die.

"struck by a sword" is killed (הרג), not died naturally (מות), right? So the bottom line is that any non-shechitah cause of death makes the animal nevelah, and there's no reversing that.