The Violi theory is a good one that has merit as there a bunch of them in Hamilton.

Im going off memory and will check exact events and timing tomm. but Mafia Inc. stated Sergio Pirccirilli (spelling could be wrong who is very close with De Vito) back around 2006 or so had a Hitman trailing Nick Sr and the guy had him lined up in the Cosenza Social club. The Hitman asked Pirchelli if the hit was a go but Pirccirilli told him to wait as he had to get approval from Ontario. Pirccirilli is affiliated with same Calabrian clan in Hamilton as the Violi's.

The reason for the hitman was Francesco Arcadi owed he and the D'Amico's money from a drug deal gone bad and efforts to reach out to the Nick Sr were not being heard. Pircccirilli was pissed and much like De Vito openly declared war against the Rizzuto Clan. If I remember correctly Nick Sr was arrested shortly there after and that may have bought him time.

Flash forward to when Nick Sr. was killed in 2010, police had spotted two of the younger in Violi's in Montreal the week before the hit talking with unnamed members of Montreal mafia.

So putting all of the above together especially with recent info about De Vito and it would be easy to make a case that De Vito, Piccirilli and Violi's have had some involvement on the hit on Nick Sr. as all had good motive and the parallels the above poster mentions seem to be more than a coincidence. It would be interesting to know if these guys at least temporarily were involved with Montagna.

Montagna always gets the blame for Nick Sr but a better case can be made for the above group of people.