I find it's rarely a simple 'corruption' like a switch gets flipped. The mage's allies usually start out compromised—they're already ambitious, desperate, or loyal to a fault. The power warps that existing weakness. Think of Saruman in 'Lord of the Rings'. He wasn't some random victim; his desire for knowledge and order made him susceptible to Sauron's influence. The evil mage offers a shortcut, a shared secret, a taste of that forbidden power. The ally begins justifying smaller evils, then bigger ones, until their original self is buried under a pile of 'necessary' atrocities.
It's the intimacy that makes it so chilling. They see the mage at their most human, maybe even vulnerable, which makes the monstrous acts feel like part of a grand, misunderstood partnership. The corruption is a slow poison administered in daily doses, not a single bite.