Thick fog, zero visibility. The pilot probably descended to see better but hit the trees on the ground. I think this solves the mystery of helicopter on fire and foul play.
I think it was a problem of spatial disorientation because of fog, compounded by the fact that a VVIP was on board so pilots may have been under pressure? We must remember the earlier plan was to go from Sulur to Wellington by road, until the last minute change. So the pilots may have had precious little time to mentally prepare & plan this specific sortie (even as they may be well versed with the route)- only speculating while meaning to cast no aspersions, please.
I don't think there was any problem with the heli- no failure is sudden- lights flash, corrective measures are attempted, mayday calls would have been received at the controlling ATC and they would have known something was amiss. In this case, the first responders were local police who were informed by tea estate workers of the mishap. Don't think the heli sent out even a single distress call. Eyewitnesses also report it was flying fine till it hit the tree- so it went from controlled to uncontrolled in the blink of an eye.
Finally, when flying in fog be clear of the hills. When flying in hills be clear of fog. This cardinal rule of flying was definitely broken here- probably because the pilots were leaning on experience to sail through. Also not aware of any instrumentation on-board this VIP chopper to alert them of ground proximity- but it is possible that the pilots who were descending to land (only 10 mins away) misjudged the altitude because of fog and not looking at the heli's altitude indicator & crashed.
Something similar happened with Kobe Bryant’s helicopter-
Let's wait for the final word from the CoI's findings on this one.