The news from the preliminary report is pretty concerning. I don't think anyone had this on their bingo card.
The fuel supply was switched off at the most critical time, when the aircraft was at its most vulnerable period in flight, but this isn't something that can really be done by mistake. The Honeywell switches which determine the on/off fuel supply to the engines are designed to need to be pulled and twisted out. It isn't like you could just knock them by accident. They are specifically built with the idea that you need some technique to use them.
At this stage it is the most likely outcome that it was done deliberately, but then if you are going to take your own life in such fashion, it is quite strange to either ask the other pilot why he switched the fuel off, or indeed respond that you didn't do it. Not that of course means one of them was trying to make it look like it wasn't the case. I am still pretty suspicious about all of this, and wouldn't exactly rule out some kind of malfunction, although it would be pretty unlikely. All I can say is that something is very off about all of this.
Need to wait for the full report, which is going to take a long time. I really feel for the families who will only have more questions than they did before, and are no closer to getting any closure as to what happened.