Here's how Sadie describes Serqet: "From a distance she looked all right, but as she got closer I saw that Serqet's pale skin glistened like an insect shell. Her eyes were beady black. Her long, dark hair was unnaturally thick, as if made from a million bristling bug antennae. And when she opened her mouth, sideways mandibles snapped and retracted outside her regular human teeth" (12.3). Ick!
The scorpion goddess sure gets creepy points in our book. She's pretty strong too, with her scorpions overwhelming Bast and forcing Zia to flee with Carter and Sadie, despite Zia's confidence that her magician tricks should be able to bind Serqet. Serving Set doesn't necessarily make Serqet evil in the strict sense of the word, but if we saw her, we'd rather call an exterminator than invite her to a tea party.