When a young grocery clerk who doesn’t give a fuck about most things, witnesses a kidnapping of a child, they have to resort to murder in order to save the young boy.

That is good feedback, thank you! :)

Maybe rephrase "who doesn't give a fuck about mot things" into a shorter sentence. Perhaps: When a young dispassionate clerk witnesses a kidnapping, he has to commit the ultimate crime of passion - murder - in order to save the young child.