the angels themselves
at the death
Last year, I wrote a post, “Judaism, Torah and Karma,” about whether the idea of “karma,” or anything similar, is present in Judaism, as it is in Indian and Buddhist traditions:
I also wrote another post, “Judaism: Karma at the Red Sea,” about how the rabbis saw the drowning of the Egyptian army as the outcome of their own actions:
“Ben Azzai said, ‘Everything is…’Measure for Measure’ …the Egyptians cast the male [Israelite] children into the River [Nile],  so the Blessed Holy One cast them [the Egyptians] into the Sea…’.” 
Along these lines, we can also note: Even though Egyptian women might have participated as “onlookers” in the drowning of male Israelite babies, it was presumably only male Egyptian soldiers who carried out the royal command. As a result, only male Egyptian soldiers are later drowned in Divine response; no women. Also, perhaps that response is moderated because Mosheh is saved by an Egyptian woman (Pharaoh’s daughter) — a member of the same royal house that had issued the command itself: “…whoever saves a life is…as if he [or she] saved an entire world.”  But at the “makkat bechorot” — the “death of the first born” (10th plague) — even Egyptian women suffered the same agony that Israelite women had felt, because they had participated, however passively, as “onlookers.”
Why do the rabbis emphasize this principle — this “point” — over and over again, throughout the Talmud, Midrash, and later rabbinic literature?
Do they mean to teach us to live every moment in depressing dread of being punished for our own mistakes?
To do so would destroy the very joy of living that is at the heart of Jewish life as taught in Torah. It would also contradict and undermine serving G-d with joy, as is also taught.
Do they mean for us to gloat at the destruction of those who sought (and seek) to destroy us?
That would contradict and undermine the rachmanut — the compassion — that also lies at the heart of a life lived according to Torah. G-d reproached the angels themselves for rejoicing at the death of the Egyptians.
For the rabbis, this may be a “cognitive” principle more than a purely ethical one.
The rabbis are teaching that when considering our own courses of action, we should never forget that we will be held inescapably responsible for the consequences.
Unlike the modern world, in which those with enough personal or corporate money (e.g. tobacco companies, environmental polluters, etc.) believe that they can escape ethical responsibility for the consequences of their choices, the rabbis remind us that this is never, in fact, the case.
What’s more, they want us to remind ourselves of this. Perpetually.
Further, they mean to tell us that when some misfortune comes upon us, despite the natural instinct to become angry, worried or resentful, we should first consider that it is (or at least might be) a Divine reaction to our own actions:
“Rava, and some say Rav Chisda, said: ‘If a person sees that suffering comes upon him, let him examine his actions’…” 
The same teaching echoes in Rabbi Morris Lichtenstein’s writings on healing prayer:
“…when one experiences the approach of ailment, he should earnestly interrogate himself: Does my mind shelter worry and fear? Am I filled with bitterness or anger? Am I poisoned by hatred or envy?…” 
The rabbis also tell us that when wrong is done to us, even if human justice is ineffective, Divine justice is ultimately undefeatable. We can’t always see this directly. Accepting G-d’s Justice must be done with the heart, not the intellect; least of all with the eyes.
Finally, for the rabbis, this wasn’t an abstract “principle” in the philosophical sense. It is a fact, around which we must form our views, actions and reactions.
Accepting that “fact” is the beginning of “receiving the Kingdom of Heaven.”