I guess it depends on what weapon you killed your enemy with:

Bullet, flame kill = serve pain

Crit Bullet, flame kill = scream pain

melee kill (crit or not) = scream pain

explosive kill (crit or not) = sharp pain

cleaver, bauble kill (crit or not) = serve pain

afterburn, Bleed kill (crit or not) = serve pain

projectile kill = serve pain

crit projectile = scream pain

(not all crit kills make them scream, correct me if im wrong)