It's alright to infect the Borg, it just wasn't alright to use an innocent and polite kid like Hugh as Subject Zero.
It's like putting smallpox infected babies in a catapult and firing them at your enemies camp is a little underhanded.
Don't use babies.
Find some cattle or old people.
(If Hugh had been an evil asshole, it would have been all above board.)
What Picard didn't do is what Icheb's parents did do which is what Janeway wouldn't do with Seven when she was infected.
Janeway had a Borg BWMD and she could have plagued the shit out of all of Borg space if she just turned around and headed back to Borg space, but going home (She went back to Borg space in Flesh and Blood... Too early still.) was more important than the right sort of genocide that would save the Federation form the next Borg Invasion.