When an ally dies.
"Oh no, they got Jimmy! My best friend! What? I thought i was your best friend! Erm yeah, you are now!"
Vendetta is an unlockable passive Ability. Once acquired it grants the Trooper 50 Initiative per teammate lost. A useful upgrade for snipers or rocket troopers due to abundance of dying grunts in one's army. Shock troops won't benefit from the ability, however.
Note that the next wave of reinforcements will get the boost for the last teammate killed in the wave prior.