The speaker has watched a cat die, apparently by lethal injection, and cannot shake the memory. The opening line sets up a small joke against belief: they don't believe in hell, yet the scene they witnessed felt close to it. The needle, the cat's twitch of panic and her long cry of distress all stay lodged in the speaker's mind. The brief parenthesis, "They said it would soon be over," sounds like a hollow reassurance that didn't match what happened.
What stands out is the speaker's guilt. They call themselves both "executioner and witness," which suggests they made the decision to end her life and then had to watch it carried out. They even wish they hadn't looked, imagining the cat alone in a "blank abyss." The closing line turns the opening around: the speaker now lives in a kind of purgatory of replayed images. The mood is raw, haunted and quietly remorseful.