I think the problem here is that you're looking at each individual action in a vacuum, instead of building a cohesive pattern of behavior. Each action taken by the Dominion demonstrated their hostile intent and unwillingness to negotiate with solids, things that cannot be ignored. While the Klingons and Romulans also engaged in some of these behaviors at various times, the main difference between them was the fact that they were ultimately individuals
, with numerous conflicting opinions that gave the Federation opportunities to find people willing to engage in diplomacy and resolve issues peacefully. Not only that, the fact that those nations were comprised of individuals fighting for individuals meant that their leadership had to be concerned about the ramifications of their actions, because it could lead to others trying to take their positions.
With the Founders, 99% (or more, all we know is ~300 infants were sent off into space) of the population is in constant contact with each other. There are no factions of Changelings, pre-Odo's return at the end of DS9; they all buy into the idea that the Dominion is the best solution to the problem of solids attacking them (which appears to be totally justified, given how the Founders insist on infiltrating and undermining every nation they don't control). They are accountable to no one but themselves, because they are at the top of the Dominion, which allows them to callously create life to be expended crushing those that don't bow down before the Dominion. They have committed acts of slow motion genocide with the Teplan Blight (which I didn't mention before because Section 31 didn't know about that when they made the virus). The Founders view everything a zero-sum game: either you obey them or you die.
In that light, the choice to genocide the Founders is a logical, if tragic, decision. If the Founders were willing to not be dicks, I bet there could've been an actual peace process between them, instead of years of cold war that eventually went hot.