The first sign that she was changing was the fact that corruption energy was dripping out from her feet.
When this happened, Zeldir had no choice but to drop Dorothy by transforming back into his small form.
It wasn't that he wanted to drop her, it was that the corruption energy had been trying to enter him. It was as if the corruption energy that came from Dorothy was some kind of living symbiot that was trying to find something to attach to.
Which was why Zeldir had no choice but to drop her or else he would put himself in risk of being attacked by the corruption energy.
It was actually worse for Zeldir since he was a god's messenger and was filled with divine energy.
If the corruption energy had actually attached to him, it would have created a large mess as it fought the divine energy inside of him. It would have basically destroyed him from the inside to let the corruption energy in.
