You should look up the bad blood that had been brewing between Crusaders and Byzantines since the first Crusade. The Arabs had trapped the Crusader army in Antioch and Alexios was marching an army to relieve them. However, on the way some fleeing people told him the Crusader army was lost and that he should turn around. So Alexios abandoned the Crusaders who in actuality pulled off a shocking victory against the Arabs. It was after this that the Greeks were known as traitors to Westerners (in addition to other things, like using Muslim mercenaries) and the Crusaders broke their vow to give back any lands conquered to the ERE and instead set up their own principalities.

In addition to this, there was the Massacre of the Latins which took place just 20 years before the Fourth Crusade. Basically the Italian Roman Catholic merchants of Venice, Genoa, etc. had been amassing power and wealth in Constantinople for years at the expense of the native population. Tensions ran high, exacerbated by both religious differences (which, if you look at Byzantine history, are a recurring theme for massacres and disorder) and the fact that Venice had been at war with the Empire. So yada yada yada, after a particularly Italian-favoring empress gets overthrown, a new Greek-pleasing Emperor swoops in and celebrations quickly turn into violence towards anything Italian or Catholic. Tens of thousands of people die and the Latins are pretty much forced out of Constantinople.

So 20 years later these same big merchant families whose relatives were massacred by the Greeks get a chance for revenge and plunder and take it. There is so much more to the answer though, you could fill a book with the reasons why Greeks and Westerners mistrusted each other.

Additionally I feel like you've missed a point in your research about the costs of the whole thing. The Venetians didn't merely want to expand their power, this Crusade had nearly bankrupted the whole damn city! They basically shut down their economy to provide the ordered number of boats for the Crusaders, only to find much less soldiers than anticipated showing up. So a huge financial investment was undertaken that now had no way to repay itself. So you have an army and need money, what do you think is the best way to get it? Sack some rebellious city on the Dalmatian coast, then when the opportunity presents itself, take the riches from those villainous Greeks!

I'm not shure why you think there had to be some kind of conspiracy at play. There was certainly a lot of mistrust between Latins and Byzantines by the time of the Fourth Crusade and Venice had some vested interrests in the east but I don't know of much evidence that the crusaders had planed to sack Constantinople beforehand or that they already wanted to partition off parts of the Empire.

The crusaders needed money and the Venicians wanted to be compensated for their ships and when both were approached by the Byzantine prince Alexios at Zara they got the chance to achieve both and more. To help the prince and his overthrowen father might have been planed in advance but that only meant to install a benevolent regime in Constantinople that would support the crusade.

The situation only escalated when the reinstitudted Isaak II. and Alexios IV. were overthrowen again and the new emperor Alexios V. denied any support for the crusaders. Then the city was taken for a second time and only then it was plundered and the decision was made to conquer the Byzantine Empire. I don't know how Venice or the crusaders could have planed this beforehand.