All you section 31 supporters do realize that the justification you are using to support them is the same one the changelings used to found the dominion?
They believe, in the interest of their security, that they are justified in doing whatever it takes to safeguard themselves. It does not matter who they have to kill or how many races they have to genocide. As long as the end result is their safety and security then it is the right thing to do.
What you presented is a straw-man.
Sci wrote pretty much the same thing in his previous post.
Why is it a straw-man?
Because the federation, other races/other countries do NOT act with blatant aggressivity to reduce the founders/your country to slavery. And it is quite unlikely that they will act so.
Meaning, genocide-level response is not necessary in order to guarantee your freedoms/the lives of billions of your citizens. Consequently, such a response is utterly unjustified; as such, utterly immoral.
In the case of DS9 (the situation you distorted with your straw-man comparison):
The dominion/founders most definitely acted blatantly aggressively to reduce the gamma and alpha quadrants to slavery - and genocide was one of their standard methods of operation in achieving this goal.
Further, 10000 years of dominion history showed that the chance of the founders stopping from this endeavor is practically zero - as long as they exist.
Meaning, in order to stop the founders - the great link, the de jure/de facto head of the dominion meat grinding machine - killing most of the shapeshifting race is necessary; there is no other option for stopping them with a significant chance of succeeding.
There is the small chance that the founders will change their ways by some other means, but, as said, winning the lottery is more probable; and if you are idiotic enough to base your acts on faith in such improbabilities, your civilization and all it achieved will disappear, and very soon - the universe is not forgiving to suicidal fools.