Character needs killin', y'say?  What kind of weapon do we want for this job?  A rusty knife worked great for one of Melody's characters.  I think everyone that dies in Alaidia gets shot... watcha thinkin'?
Okay, now that I've thoroughly disturbed everyone... 

My first thought.  What is the relationship between these two friends?  What are their genders?  Did the victim sense that it was coming?  All of those factors will help you decide what kind of reaction make sense for them.
If a girl were betrayed suddenly, she'd probably be in panicked "How could you?" distress.  A guy that knew what was coming might be more of a "die bravely" kind of stoic guilt approach.  If the bond was deep, you might see a lot of sorrow from the victim for the killer who has turned on him.
Try envisioning a different scenario that involves the same emotions.  What if these characters were sitting at a table and the killer told the victim he betrayed him in some way.  How are the two going to react?  Then take that knowledge and translate it to the killing scene.
My other suggestion?  Keep it short.  Only show what you need to.  What kind of information do you need to get across in this scene?  If all you need to establish is that a friend killed his friend - and his friend realized the killer's identity - then you could do that all in 3 lines a la...
Quote:
He knew it was me.
All he said was, "I loved you like a son, Anakin."
And then I pulled the trigger.
And that's all you need.  Impacting, short, but informative.
Hope that helped!