Because he didnt want to die anymore he said he wanted to destroy the world or someshit
I think zeref fedup bcos the world keep rejected him or he don't to die bcos he don't want his young brother to die or zeref have his own agenda??? Hmmppp
It's because he doesnt want to die without telling natsu the truth, especially since natsu is a demon from th book of zeref that would die with him. Might also b effect of the contradictory curse he has. Though i think the chapter is pretty much anti climatic given how all the information was given just like that.
But I kinda get the feeling that if Zeref didn't say anything Natsu would have been able to finish him off. So if Zeref really just wanted to end his life, why did he confuse Natsu and Happy and make them hesitate by telling them all this ?