Hi Johnny Lim,
As the Buddha said “cetana ham bhikkhave kamman vadami”, please read “What is Intention in Kamma?“, basically means kamma is intention (cetana). So if one didn’t have any intention to harm but accidentally killed another being then it doesn’t have any power (javana) to bring about vipaka. What make one remorse is just simply one’s avijja (one doesn’t know about the general working mechanics of kammas)
Further more, there is two general rules to calculate the seriousness of the kamma:
1) Which of the dasa akusalas (or kusalas) the intention?
2) Then the strength of the kammā vipākais based on the “level of consciousness” or “qualities” of the living beings affected by that kammā.
The weight of the kamma also depends on how many of the necessary steps were actually completed. Just having an intention is not enough to yield the full impact.
As per my observation and contemplation, most of animals don’t have enough capability to revenge, why is that? because they mostly go with the flow, even if they were killed then their gandhabba will just go out and aimlessly wandering until be pulled toward a worm, or if their bhava kammic energy is exhausted, they will most likely be reborn an animal again. Let alone they were killed by accident which means they had not acknowledge the killer. Of course there are some special cases which their mind were calm down at the moment of death so past good vipakas can come to play, like the frog later become a deva just by listening to the smooth voice of the Buddha. But this kind of advancement is extremely rare.
If we do not take the chance of knowing pure Dhamma and make a determination to get out of Sansara, we will most likely helpless like the animal, I think this is the core of Metta bhavana, to know that every being are on the same boat with us, aimlessly wandering and do not acknowledge the truth of life. So compassion naturally arise and with true & pure intention. So it mostly for oneself that matter, anatta (helpless) is a characteristic of life, so it’s better to get out of the sansara ASAP without let the external environment have a bad affection on our progress toward Nibbana.