Daphnes, wouldn't that just mean he did what he had to do, once the Yeerks started winning? Aldrea and Dak kind of seemed to be sabotaging something that would have worked.
Now I'm not saying what Alloran did was "right", but nothing in war is right. The Hork-Bajir were as physically dangerous as the Andalites themselves, allowing the Yeerks to expand and take more species.
From a certain point of view, committing genocide on the Hork-Bajir would have saved more lives overall than it took, from a variety of races.
Plus, just throwing this out here (fully expecting a backlash, I know), that the Hork-Bajir weren't exactly a "natural" race, anyway. Experiments of the Arn, somewhat of a gray area in itself as to whether or not they should even exist. Although I can't remember if Alloran knew of their origins at the time, haven't read the book in a while.
But, all being said, if Alloran's ruthless decision had just been supported from the start, he was right, more good would have come of it than bad. The Andalite High Council didn't have anything to do with it, remember. He broke the law, but for a pretty damn good reason.
And it seemed the Council learned of their mistakes by the time of Earth, their quarantine plan seeming to hint that they finally realized just how cold and harsh and extreme things sometimes have to be, that perhaps Alloran was right all along.
The Andalites couldn't lose the humans to the Yeerks, simply couldn't, just as they probably shouldn't have let themselves lose the Hork-Bajir. Without the Hork-Bajir, the Yeerks would have been relatively easily overrun and defeated. The Andalite unwillingness to exterminate them fueled the war for however many decades it lasted. Alloran knew this, and was persecuted for it.