Very strange.
The most significant evidence seems to be the crime scene and the story it tells as to how the killing went down. Either from Zainab's (the 7 year old child) account or somehow based on forensics, it is believed SM was the first person shot. Apparently there was blood splatter from SM on the shoes of both Zainab and Saad Al-Hilli. SM was well off of the roadway when he was shot. He was "interacting with Saad. 'Saad was then able to jump in the vehicle pull forward, apparently running over SM and then backing up, most likely to align the vehicle so that he could pull out onto the road and escape. When PM arrived on the scene, the vehicle was backed into the dirt and the wheel was spinning. Most likely, Saad had lost consciousness while backing up.
It has been reported that 21 shots were fired from 3 clips that held 7 rounds each. It has been reported that few if any of the shots missed their mark and PM was shot 5 times. We don't know where and how many times the others were shot but the fact that all of the adults were dead when PM checked minutes later suggests multiple head wounds. Anyone who has experience with a handgun knows that they are very inaccurate, even at short range, unless you know what you are doing. This guy did.
There seems to be no getting around the fact that the killer wanted both SM and all of the al-Hilli group dead. Had he been a Hit Man targeting just SM, it is inconceivable that he would have killed an entire family of innocent bystanders who happened to pull into the intended murder site. He would have driven his bike up the road a bit and found a different spot. Killing 5 people is far more problematic than killing one. Had the target been the al-Hilli's, and a French bicyclist just showed on the scene, Perhaps.... It would require pretty quick thinking but it is possible.
If it was a Hit Man targeting the al-Hilli's, how did they know they would be at that particular spot at that particular time. (the forester reported the motorcyclist/killer arriving 15 to 20 minutes before the al-Hillis) If they were spending the day exploring the countryside, it is highly unlikely that they had any specific plans to be at that spot at any particular time, and equally unlikely they would tell anyone about it. (and certainly not his brother)I see no way anyone could have just "figured out" that they would be there around 4:00.
I think the most likely explanation is that al-Hilli had set up a meeting with someone, possibly SM or even the hit man. Someone knew about it.
Why bring his family? Possibly to use a holiday with the family as a cover whatever he was really up to. He had an appointment a the Swiss bank the next day. That seems like a lot of trouble to keep his brother from getting wind of it. (as I pointed out earlier, Zainab was pulled from school to make this trip)
From what I can tell, the brother was definitely trying to defraud Saad out of his inheritance. This could be a motive but would the brother have the contacts to find a hit man who could pull this off with such a degree of proficiency? We don't know that much about the brother.
If the family/money issue is ruled out, then we are left with a very strong possibility that this is international espionage; spook stuff.
al-Hilli was just a engineer who did contract work for communication satellite company with no security clearance and no access to classified material. (isn't that what they all say?) He did have in his possession, a lot of scientific information that was far beyond what he would need for his work. Was he up to some nefarious activity. He was known to rabidly hate Israel and had some contacts with Iran including a resent unexplained trip there. Could he have been trying to recruit SM to work in Iran? It isn't beyond the realm of possibility.
If this something to do with international espionage, anything is possible and we will probably never find out. Certainly a heart attack on loose end in America could be arranged.