I texted my guy friend I will kill you and he texted back "Just kill me uff", what does that supposed to mean when he said like that?

we were sitting in the classroom and he was taking a video of me, so I texted him that I will kill u and he answered like that then after some minutes he looked at me and texted pretty. what does that mean and why on earth does he try to make me mad?

I texted my guy friend I will kill you and he texted back "Just kill me uff", what does that supposed to mean when he said like that?
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