Remember how in FF1, Garland is killed but is sent back in time 2000 years by the Four Fiends, and the Fiends give him their power and then Garland uses that power to create the Four Fiends and send them into the future so that when he's killed, they can send him to the past

Xehanort's plan is too complicated though

Remember how in FF2, The Emperor allowed himself to be killed so that he could go to Hell, kill Satan, become the ruler of Hell, and then resurrect himself with exponentially more power than before

Then remember how in the GBA version, it adds another facet where when he died, he split is soul into two parts, light and dark, his dark half going to hell and his light half going to heaven, so that he rules the entire afterlife

Final Fantasy just always been like this, this isn't anything new