Thorn is the only gun in the entire game that can kill someone after that person has disengaged.
Boom
I'll let you recover from your blown mind now.
Better?
No matter how far you run away or how much you try and hide, if you've been tagged by Thorn the magical amount of times, YOU WILL DIE.
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The same thing will happen with a regular gun and burning grenades. It's just more convenient with thorn.