One. Four. Five.

J and K are good friends/partners/lovers

J finds out that he has an unknown enemy

J discovers evidence that falsely accuses K

J believes that K might be the enemy

J confronts K

K rightly denies all accusations

K feels relationship was not good enough to disprove accusations

K feels betrayed.

The sad thing about the scene is that the feelings and beliefs usually seem genuinely justified. It might have been because of very convincing evidence that led J to believe that K could be the enemy. And if J could honestly believe that, then its only natural for K to think that their relationship was not sound, hence the feeling of betrayal. 

But is K's reaction justified?  I don't think it is. 

It is important to recognise that there are actually two betrayals - J actually felt betrayed first. Although it was because of false evidence, J nonetheless genuinely felt that K betrayed him. Hence, if K feels that he is justified to feel betrayed at the end, then J should similarly be justified to feel betrayed at the beginning, therefore justifying J's consequential belief, thus in turn nullifying the justification of K's feeling of betrayal.

Reductio ad absurdem - K's feeling of betrayal cannot be justified.