While the Super Saiyan Rage transformation is necessary for Future Trunks to be the one to land the killing blow on Fused Zamasu, it's never properly explained, or foreshadowed in the slighted.

My Reviews Are Moreso Just Non-Formal Descriptions Of My Thoughts On Things I Like And/Or Dislike. I Don't Really Care To Be Formal About It, I Just Like Putting My Thoughts To Words ...