Would you describe Delia’s actions at the end of the story as revenge?

It is possible that Delia's behavior could be interpreted as revengeful behavior. However, we must also remember that Delia is very much afraid of snakes. So, it is quite possible that her fear prevented her from entering the house to help Sykes. Also, Delia likely felt little motivation to help an abusive husband. In all, a combination of fear and her loathing for Sykes prevented her from helping him.
Note that it was Sykes who originally brought the snake into the house. Initially, the snake is kept in its own wire mesh box. When Delia begs Sykes to take the snake away from the house, he refuses. Instead, he unfeelingly announces that the snake will leave when he gives the go-ahead. Delia flees the house, however, when she discovers the snake in one of her clothes hampers. It is likely that Sykes made the transfer without Delia's knowledge.
Because of her deep fear, Delia hides in the hay barn. She is so frightened that she suffers a panic attack. Eventually, Sykes returns. In the night, he gets bitten by the snake and lets out unearthly screams of pain and fear. Delia doesn't enter the house, despite hearing Sykes scream her name. The last paragraph suggests that, while Delia was afraid, she never had any intention of helping Sykes. Here's the proof:

She could scarcely reach the Chinaberry tree, where she waited in the growing heat, while inside she knew the cold river was creeping up and up to extinguish that eye which must know by now that she knew.

The last sentence suggests that Sykes knew his wife ignored his calls for help. So, it is very possible for Delia's actions to be interpreted as vengeful behavior.