The teacher, whose wedding was fixed for September 8, was hacked by Seegan, who allegedly made several proposals to her which she had turned down, police said.
Angered about her wedding with another person, the accused entered the church where she was praying and allegedly hit in her head, neck and shoulders, police said.
On hearing the woman's screams, her colleagues came to her rescue and rushed her to Tuticorin government hospital, where she succumbed to injuries, police said.
Police later found Seegan's body hanging at his brother's house.
The teacher's relatives said they had made several complaints to the police about the woman's harassment by Seegan, but "no action was taken".
A video report from a regional news agency: