I realize to you Northern types, "honor" isn't a valid concept.
I will explain: Lincoln snubbed their offer of peace, therefore, they were bound by their code of honor, to reduce the fort, or be looked upon as cowards. Even the method of negotiating the surrender of the Ft. was done in an honorable manner. They were notified that unless the Ft. was surrendered by a particular time, they would begin bombardment.
Of all the "it was all Lincoln's fault" arguements I've heard that has to be the most ridiculous.