Alloran's decision gets slammed a lot, but...

[Fwahm]

Think about a situation where Alloran had not released the quantum virus on the Hork-Bajir.  The Andalites had already lost that planet, so the Hork-Bajir would fall like normal.

However, this time, the Yeerks would have plenty of hosts to go around (based on inferences, probably in the hundreds of thousands to single-digit millions, planet-wide).  The whole reason for attacking Earth was because the vast majority of Yeerks were hostless.

With that many battle-ready troops, the Yeerks could mount an assault on the Andalite Home World, and possibly win.

The virus may have been essential to their victory, despite how much everyone denounces it.

[Chad32]

Part of it is because he screwed up from the beginning, and if he had done things right the Yeerks would be defeated there. Another thing is genocide is just not looked well upon. I mean, everyone knows the holocaust was bad, right? This is similar to that, in that he's trying to destroy a species, and not even the offending species. They made a virus to attack the Horks, not the Yeerks. They screwed the Horks over twice, and the Horks did nothing to deserve it.

[Serraph105]

you have a point, but I think the point is when you have to ask yourself how evil must you become to defeat your enemy and once you do will recognize what you did was evil or will you see it as "having to do what was necessary" and teach future generations that such actions are ok and justified.
You could very well change the way your entire species sees war with that single act of desperation.

[anijen21]

I'm pretty much with you. Genocide is terrible and all but if you're seriously fighting an enemy with a vendetta that goes all the way to kidnapping and enslaving your populace's bodies without your consent and essentially terminating their civilization, morality gets tossed out the window. Survival will always trump morality. Of course I still believe Alloran didn't intend to release the Virus, and I still don't understand why he designed it to kill Hork-Bajir instead of Yeerks, but when the Yeerks complained about the Andalites using the Quantum Virus I was sort of like "...are you effing serious WHY DON'T YOU JUST GO TO THE UN AND FILE A COMPLAINT OR SOMETHING YOU HYPOCRITICAL WANKS" god.