No, Nicholas does not think that this aunt is the Evil One. He probably thinks she's evil, given that she's always telling him off and given that she prevented him from having a nice day out with the other children. But does that make her the Evil One? No, not really.

The ever-mischievous Nicholas is just teasing his aunt. She's just fallen into an empty rainwater tank and calls out to him for help. But Nicholas is in no mood to offer any assistance; he's enjoying his aunt's predicament way too much. At first, Nicholas responds by reminding his aunt that she said he couldn't go into the gooseberry garden. Yet that's precisely where he'll have to go if he's to help his aunt. His aunt responds by telling Nicholas that he now has her permission to go into the gooseberry garden.

But Nicholas isn't finished toying with his aunt just yet. He cheekily tells her that her voice doesn't sound like his aunt's; it could be the Evil One tempting him to be disobedient. Apparently, his aunt is forever telling him that the Evil One tempts him and that he always yields. Well not this time, says Nicholas.

Nicholas doesn't believe any of this nonsense, of course. He's just throwing his aunt's words back in her face to make her sticky situation all the more uncomfortable for her and all the more wickedly enjoyable for himself.