Underworld Vampires & Werewolves SPOILERS

[Tanager]

...and her narration at the beginning "We got good at killing them, almost too good" IMPLIES that she was recollecting on a conviction to kill that she would later come to believe was in error.

Have to disagree here, while I can't bring up the exact context from the film I felt that when Selene said this it was more in relation to the fact that the extinction of lycans would make her obsolete, her role as a "death dealer" was a defining factor in her identity. I don't remember the exact quote but doesn't she make a statement directly comparing the death dealers themselves to the old, obsolete weaponry?

Otherwise, I gotta agree with the general statement about having to read into film. This film in particular seemed to rely on implication a lot more than direct exposition.