SHIFTY SPOILERS
Magnus knew Trevor had mugged Shifty of the coke because Glen told him.Magnus also knew that Glen was ripping him off by cutting the coke because he heard Glen admit in his confrontation with Shifty (Shifty had his phone connected to Magnus).Magnus done Trevor as a thank you to Shifty,hence the 'say sorry' video camera clip,also retrieving the stolen coke at the same time
END SHIFTY SPOILERS
That explains so much. I didn't click on to that at all and the ending therefore made no sense.
