Kerrigan didn't choose to be Zerg, Arcturus decision made it happen. This is on whom that choice should be explored and developed: Relationship between Mengsk and Kerrigan in terms of Kerrigan paying the price and becoming Zerg.
If she dies, it becomes the finality of Mengsk decision, and so will tear apart Raynor as it relates to Raynor vs Mengsk in the realm of losing a loved one.
If the focus was on Kerrigan alone, and she died, it's a tragedy story. One that she is a victim of a power she absolutely have no chance of resisting, defeating, and will center on the issue: Trust.