Minho: Why are you helping us, Gally? I put a spear through your chest.
Gally: Yeah, well... Nobody's perfect.
Brenda: Am I missing something here? This is the same girl who betrayed us, right? The same bitch?
Gally: I like her. (00:55:20)
Gally: You guys are nuts.
Answer: It's not so much that the rebels gain anything by destroying the city, it's just that they are untrained and driven by rage. They do not act as a military unit, they act more like a mob. They lack the discipline to strike only enemy combatants and instead cause chaos in the entire city. The point is that the rebels are acting no better, and in fact quite a bit worse, than the infected do.
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