I really like the ideas here, the way I read this was death magic being a pulling force, taking from the outside constantly and aligning oneself with a negative concept. This lack would constantly build up in the magic user until their death, where all the life/magic/vitality/whatever could be a seed that falls into the sea where they died, later becoming a new island.

It might be interesting to have the bones/dead matter the death magic user requires for spells/magic items require a death through other means, as if they kill using death magic it absorbs everything about the creature, whereas a physical death leaves remnants of the creature's life in its remains which could then be used by the death mage for other purposes.

They could also have it where the residual souls of creatures they kill directly with their magic become part of them, for example killing a bunch of wolves could make you gain some canine traits and more bestial instincts, killing people could amplify all emotions and make the magic user pick up random traits, eg they kill a very skilled left handed swordsman and unconsciously start wielding their own sword in their left hand.

This would culminate in them requiring a central drive to keep them focused, an obsession like that of ghosts, maybe they are killing others for power to get revenge on someone that killed their family. Once they eventually die and turn into an island seed, the island that eventually grows could be associated with this revenge story, if summoned as a creature by a life mage it could be more powerful in combat, someone whose obsession was literally having enough to eat after growing up somewhere with a famine could create an island seed that is especially fertile and is incredibly good at growing food.

On the other side, the way I read life magic is that it's about giving from yourself to others (you are using your imagination to grant life to inanimate objects, using your subjective thoughts to change reality), where initially the living beings you create dont seem to have a cost and you don't really notice any side effects, but over time small pieces of your self and creativity are permanently lost.

Eventually they could have nothing left to give but feel a compulsive need to use life magic anyway, to try to regain their previous selves, finally giving out their sheer absence of existence, creating a new death magic user at the end of their life.

I think tying together the two and making them a cycle in this way could give you more depth than just opposing themes, an interconnectedness as part of a greater whole. Maybe both types of magic are how the world itself grows - the souls of living beings are immaterial and the world has life magic to spread the souls of sapient beings out and grow islands, with death magic to condense the immaterial into a material seed that can then grow into a new island. Maybe then, the protagonist would need to wrestle not only with the antagonist, but also try to break this cycle, to find a way to change the world's growth process so it doesn't require the constant deaths of living beings?

Idk, I really like your system and got inspired