^BRAVO! 
Now...
Y'know, it's funny--the fact that they had a cure in the first place gives me pause.
What was the purpose of the cure? Was it a "just in case"--a backup plan for something unfortunate happening (such as Odo's contraction of the virus)? Or was it for the purpose of blackmail/bribery--i.e., offering the cure in exchange for a surrender? Perhaps both?
(Also, the fact that Sloan was captured SO easily in "Extreme Measures" implies, for me at least, that there was something going on then, wheels within wheels. Was it possible that Sloan planned to be captured, and to fake his death, so as to preserve plausible deniablility? Just saying....)

Now...
Using Odo was deplorable and that was Section31's dirty deed. There probably wouldn't have been another way to introduce the disease to the Founders.
So it was either use a Federation ally to infect the Founders with a lethal disease from which he'd probably also die- or let the Federation be conquered. You lose either way. If you infect Odo you have lost sight of what you are fighting for- to keep the Federation and its ideals and principles alive. Infecting Odo betrays those principles
Therefore I think Section31 was not justified in its actions.
Y'know, it's funny--the fact that they had a cure in the first place gives me pause.
What was the purpose of the cure? Was it a "just in case"--a backup plan for something unfortunate happening (such as Odo's contraction of the virus)? Or was it for the purpose of blackmail/bribery--i.e., offering the cure in exchange for a surrender? Perhaps both?
(Also, the fact that Sloan was captured SO easily in "Extreme Measures" implies, for me at least, that there was something going on then, wheels within wheels. Was it possible that Sloan planned to be captured, and to fake his death, so as to preserve plausible deniablility? Just saying....)