Is "everyone else was doing/would do it" really a defense? Not too long ago, one could've taken the same attitude toward the institutionalized killing or imprisoning of any number of minority groups (lets say homosexuals or Jews, which still takes place in less developed areas of the world).
Also, Tali's comment is pretty ridiculous, with her coming from a culture that she explicitly states in the previous game to have abolished the death penalty (yet another example of her highly inconsistent writing). Scratch that, it's ridiculous from the perspective of 20th century earth culture, where people afflicted with HIV/AIDS aren't imprisoned or randomly murdered because they could effectively kill someone by having sex with them.
Not disagreeing that there's a legal and even utilitarian defense Shep could use (though the choice categorically ends up failing on the merits of the latter), but that doesn't mean the asari weren't acting like the type of religious nutjobs who burn witches (the term Ardat Yakshi even translates to a type of supernatural "demon"). Saudi Arabia has a legal defense for the stoning to death of women who get raped, but that doesn't mean it isn't barbarism. Personally, I wouldn't support taking part in such actions if asked.
Of course, the whole premise of the mission is so full of stupid to the point it's hard to take seriously as any kind of philosophical or social commentary. If the asari just wanted to destroy the prison to prevent its use as a banshee factory, they could have simply glassed it from orbit. Same goes for Shepard and the Normandy.
The perfect analogue for Ardat Yakshi would be vampires(also known as demons *wink**wink*) not HIV/AIDS sufferers, with the added benefit that their condition doesn't actually harm them, apart from being sterile I guess. The Ardat Yakshi gene is an addictive one which makes the Asari more compelled to mate even though they know it will kill the partner (basically like Vampires being addicted to sucking blood even though it kills the person they're getting the blood from), and each time they kill they get smarter and stronger (like vampires do when they suck blood), something which only servers to feed the compulsion and make them seek more people to mate with and kill. Finding someone to kill also really isn't a problem for them since they can seduce just about anyone, and can take control of entire villages and make them their thralls (once again, another vampire ability), meaning that they can rack up the kills and like Morinth did, reach the power level of Asari Justicars when they're barely out of their Maiden stage.
So now lets take a look at this again, does it really come as a surprise to you that most races would outright kill a group of people who are literally Space Vampires? Because we humans sure damn would, without thinking. They aren't just people with a simple (I say simple, but you know what I mean) illness like Aids, a single one of them can decimate an entire population area and leave it full of husks. Now you should see why Tali is completely correct in saying that the Asari were generous in giving them a chance, because barely anyone else would. Like Is said, no one in the Mass Effect Universe is going to shed a tear for the Ardat Yakshi, or as I will call them from now on, the Vampires, Liara is alone on this one.
Fun Fact: The ship that Morinth escapes with from llium in ME2 is called the Demeter, the same name of the ship that Dracula used to get to England. Bioware didn't even try to hide the vampire connection lol