Completely lost

Have wanted to see this for a long time and watched it tonight. I really don't get how it progressed. How did Mark and John's relationship start to dive? It seemed to be when John turned up at the house and found them all taking a break? As if he lost confidence in Mark at that point. And I'm completely lost as to why John killed Dave. John was determined to have Dave on board, then later calls him in ("no matter how much it takes"). So why did he kill him? Jealousy? That Dave had a better relationship with Mark than him? That John was incapable of forming strong relational bonds?

I don't mind a slow burn movie, but this was so understated I found it frankly confusing. I don't usually need a movie explained, but I do on this one!