Gulab Singh (55) got into a heated argument with another guard Ramesh Singh, following which an exchange of fire took place between the two, Circle Officer R K Chaturvedi said.
Both Gulab and Ramesh along with another security guard, who was standing near the spot, were grievously injured and rushed to a hospital in Lucknow.
"While Gulab succumbed to his injuries on the way, Ramesh and the other security guard are undergoing treatment," Chaturvedi said, adding, an FIR in this regard has been lodged with the Amethi police.
Rajesh, who was Samajwadi Party's nominee from Amethi in 2007 Assembly polls, was expelled in July this year by Chief Minister Akhilesh Yadav for anti-party activities.