In the Nod campaign, Umagon's first appearance has her Neck Snap a Nod Mook. She later kills 2 more when she escapes from Nod custody. Ironically, she doesn't kill anyone in the GDI campaign cutscenes.
In the GDI campaign finale, despite being weakened by a Tiberium serum, she manages to throw off Kane's chock-hold on her, allowing Mack the chance to attack Kane.
In Firestorm, the Enemy Mine between GDI and Nod to take down CABAL.
Hassan's execution. Not the execution itself, but the revelation that Kane lives.