Question

What role did merchants from Venice, Italy play in the decline of the Byzantine Empire?

Answer 1

Well, I know that the merchant's trade rought avoided the Byzantine Empire, thus they declined in trade, and as a nation.

Is that a sufficient answer? Or do you need more detail?

Answer 2

The Fourth Crusade was the most direct result.

The Byzantines had antagonized Venetian merchants by trying to break their power in the past, and the Fourth Crusaders at the time were in need of ships and transport to get to the Holy Land -- which the Venetians were all too happy to bankroll and which also gave them tentative control of the Crusade.

Another thing is that the Venetians, led by Enrico Dandolo who had suffered at the hands of the Byzantines years earlier and who led the Crusaders, convinced them to eventually support a new emperor to the throne of the Byzantine empire. The reason was that the Crusaders were heavily in debt to the Venetians who backed their efforts and that the Byzantines would support them -- if only they would support the bid of a new emperor to take the throne.

So they sailed to Constantinople, pillaged it, placed their puppet on the throne, and actually made the city into part of a Latin empire for a time. It was a historically humiliating and horrific event that further drove the Roman Catholic and Eastern Orthodox churches apart.