There is not much to say other than I think if an enemy is under the effects of head pop and glowing blue if that enemy dies from anouther source then it should count the same as a headpop killing it for the psyker.
At the moment we have this odd mechanic where we need to hold off attacking the glowing enemy which seems counter preductive to me. It would also allow traits to act during the period of the headpop as a sort of debuff which makes more sense, rather than on activation of the headpop.
At the moment we have a mechanic that essentially makes an enemy stand out that we need to stop attacking, this seems counter productive and can only be learned by a player picking up pskyer themselves and becomming fustrated by how often other classes dont get this, and who can blame them? It runs counter to pretty much decades of videogame logic.