“Pāṇā” means, living beings.
You could think of the first precept as a circle. At the middle is the stuff which everyone agrees is obviously very bad, illegal, or morally reprehensible. Toward the edges are the things on which opinions could possibly differ. Most of the pacittiya-type offences are generally closer toward the “edge” of the circle. Although the texts do not have a concept of an offence of aggravated cruelty to animals, there are certain things one could do that would make the offence weightier, after the full scope of monastic jurisprudence has been applied, as Bhante @Dhammanando has mentioned.
None of the things in the circle of killing are things which you SHOULD do, however, as you get closer to the edge of the circle, there may be things that you HAVE to do, on account of lay livelihood.
If you are after more social context than that, I sometimes moonlight as an armchair brahmin lawyer, but I don’t know what is useful for you. Hindu ethics and legal thinking at the time of the buddha was for the twice-born, most of the texts detailing codes for the working class came much later.
By contrast, what happened in the medieval period in Jainism, for example, was that a whole class of literature emerged attempting to give detailed lay moral codes for lay Jains as a general class. Buddhism never quite developed such an explicit set of literature, so we have to borrow reasoning based on the vinaya (or occasionally on the basis of secular reasoning) as a way of shining a Buddhist light on the details of lay precepts. There have been a number of debates as to the true source of Buddhist ethics, i.e. whether these proceed from the secular law, or whether the secular law should proceed from Buddhist ethics, however, as they occurred in Burmese, I don’t have original sources. The Pali texts themselves highlight the secular and universal nature of the first four precepts, for example, defining murder as being “murderous and bloody-handed”, and lying primarily in terms of perjury or serious lying. If you break the first four (edited: remorselessly), you are not just not a Buddhist, you have failed in some respect as a human and are going to go to hell. See examples of further definitions given at SuttaCentral AN10.211 Hell.
There are some Buddhist texts (Dharmaguptaka vinaya commentary) which suggest that it is not strictly required for a Buddhist lay practitioner to never ever kill a living being. T. No. 1804, 40–2, 77a15–21, in the Chinese Tripitaka, actually suggests that the removal of pests is a duty of a kappiya (the lay attendant helping out the monks). A lot of lay livelihoods require various types of harm to animals and minor killing, like digging the soil, cutting plants, etc, but these aren’t an absolute barrier to Buddhist lay practice unless you are actually a butcher. For certain classes of people, like kings, their business of state involves punishments of even human beings, but they are still able to be Buddhist lay people.
My personal opinion is that what is strictly meant by the five precepts is that one won’t break the law (the four precepts), + alcohol, which is specifically for Buddhists (i.e. as much as I would love to see alcohol bans, it isn’t required for Buddhists to lobby for them in general). Keeping in mind that what was meant by “the law” at the time had a deeper moral component than what is meant today, with adultery and some forms of extramarital sex being illegal. I haven’t asked my teachers directly about this, but I was given “updated” materials where Buddhist sexual ethics have been modified to include concepts like consent, so we can see that the secular law is very influential in this respect.
Serious lay Buddhist practitioners will often confess, and may even ask to retake, precepts they have broken. It is not uncommon for lay residents at a monastery to attend a fortnightly ceremony of taking precepts that mirrors the monastic one. If this is done, the purpose is freedom from remorse and individual training. You would do something like that to help get your mind still enough for meditation and to practice for the future. It’s a bit different to the equivalent concept of purification in brahmanism, which is about pollution/purity and caste status.
One time, at a temple I was attending, a supporter grabbed the bins and sprayed the maggots (dead) in front of everyone. While this sort of thing is distasteful and likely very unnecessary, no-one was arguing that this person should have their temple membership revoked on account of being a flagrantly precept-discarding non-Buddhist. In some respects, they (arguably) did the temple a favour, by doing the thing the rest of the upasikas wouldn’t do. I think it’s also possible that they could train to be more mindful to deepen their commitment to the first precept if they chose to do so. HOWEVER, if they killed a human and weren’t remorseful, yeah, I think that would be grounds to revoke their temple membership due to not being Buddhist.
While everyone wants to see animals protected and less killing, sometimes preaching at temples can go in strange directions. YES devout Buddhists love animals, but if you wanted that to be your whole religion, you could theoretically join PETA or the Jains who are doing a more thorough job at it. I don’t think the success of the Buddhist religion as a whole rests on the fact that individual lay practitioners are doing a great job at not swatting mosquitoes (although it’s great that many people do avoid doing this). Once, at school, I had to sit out of tennis in physical education class because there were ants on the court- I sat out of so many things, including the toad lab in university- and while I was happy to do so, I could see why equally sincere Buddhists would choose not to sit out, given that things like one’s education and livelihood are equally legitimate lay concerns.
Like I said though, some of these issues have been debated. While I believe I’m representing the literature to the best of my ability, others might legitimately have different opinions. I can’t ever tell anyone that there are no kammic consequences if you do choose to play tennis on an ant court. What I can tell you is that Buddhism is a magnificent religion and offers so many chances to do good that in the long term, the choices of individuals to undertake livelihoods that include minor harm or minor killing may not be the deciding factor for your post-mortem fate or your journey in samsara.
