If it's true that they started at 1800 the day prior then I would agree 100% had fatigue was the main factor. Some of you might remember the DC-8 freighter crash at Gitmo(Cuba) many years ago. The crew had been on duty for 16 hrs. They were so tired that they didn't care. They were highly experienced and had been to Gitmo before several times. After that crash the rest rules were amended by the FAA.
The DGCA had some rules back which were more "pilot friendly." But bowing to pressure from the airlines they went back to the old FDTL rules.
Remember, being legal and being fatigued are two different things. When airlines expect or suggest that you get 8 hrs of sleep right when you get to a strange hotel in a strange land and a different time-zone, they are severly misguided. A BA Skipper in Delhi a year or so ago called in fatigued for his flight to London because he was not able to sleep in his hotel because of a party/wedding that went on all night long. He did the right thing but strangely all the passengers he would have flown were up in arms. I guess they were Ok with a dead tired Captain flying them to London. Everyone talks about safety etc, but then............
My understanding is that AirIndiaExpress is the biggest culprit in violating DGCA FDTL rules.