They created it to be a mediator, the Catalyst decided that wasn't enough and a more drastic solution was needed.
yea but they also created something with that ability. it's not like someone can build a car and than the car just starts flying.
The Quarians created a car, a car that later flew. The Geth were never intended to be smart AI's, the Catalyst was never intended to do what it eventually did. The Geth are apparently the only AI aside from EDI that didn't turn on their creators in however many millions of years.
One of the things that bothers me the most is that when the Leviathan's thralls created AI that turned against them, those AI's were defeated. Given the fact that they can take on Reapers...well, the creation of an AI to mediate reeks of laziness when crushing a few (supposedly inevitable) rebellions every now and then would have just meant a bit of effort.
The Quarians created a car, a car that later flew.
no they didn't, they created a vehicle that was waterproof so that it would function better in the rain than they flipped out when people drove straight across the river instead of paying the tolls on the bridge.