If they do evil things now and then but apologize and repent for some of.it and even do an occasional good things... doesn't that make them neutral? (What else would neutral be?).
It's not
quite in line with the topic, but my own opinion is that alignment should be less what you are and more what you're
aligned with, and it should be vary in strength based on degree of separation from the source of alignment. The further from the source, the more likely for there to be a neutral element on at least one axis, with full neutral simply being "unaligned."
For example, say you have an evil god. The god and their direct representatives(extraplanar in nature) would be
intrinsically evil(LE, CE, NE). Their proxies (faithful clerics and champions) would be
strongly aligned with evil. Warriors to the cause, mundane adherents and evangelists, etc would be
moderately aligned with evil, and your average citizen, tribe member, etc. would be
weakly aligned at most. It wouldn't have to be religious in origin either. Sufficient dedication to a specific cause or being could qualify, though some connection to an extraplanar or supernatural origin would be necessary for an intrinsic category. A cabal of evil mages(strong), most undead(intrinsic), the upper echelon of a slaving organization(moderate), etc. Anything that represents a strong and enduring dedication or association to an evil cause or origin.
This wouldn't represent an "okay-to-kill-ometer," but would mostly reflect two things: what it takes to turn them from their alignment and how alignment specific spells would affect them. Intrinsic alignment cannot be turned away from their alignment through mundane means if any at all, and, for example, a
protection spell against their alignment would stop them outright. Strong alignment would be nearly impossible to turn away, and the above spell would inflict a penalty to the affected person's attacks. Moderate alignment could be turned away by extended separation from the source of alignment or extreme hardship such as starvation where the source of alignment is perceived to either be responsible or failed to have prevent it (though propaganda can easily shift blame in absence of an opposing viewpoint), and weakly aligned are only aligned at all because they're told they should be, and can change (but won't necessarily) by the introduction of a convincing and opposing viewpoint.
A system like this would introduce the kind of nuance people want without discarding a useful tool. In your example under this system, the question wouldn't be what they did, but why, because alignment isn't a behavioral straightjacket, but an indication of goals and dedication. A man who kisses his wife, plays with the kids, and then goes to the ruined temple to work on the ritual to summon Ithpaxt the Merciless is of evil alignment, not because he always chooses to do evil things(hard to get along with your neighbors that way), but because he is dedicated to an evil cause. The woman who murders her husband for selfish reasons is, barring other factors, not evil, because murder is an act, not a cause. And it would trickle into the makeup of societies. Isolated societies with a monolithic religion would trend towards a specific alignment (see the drow issue), where more cosmopolitan societies with a more polytheistic bent would trend towards neutrality among the less dedicated. It wouldn't mean that the drow themselves are evil, but rather that they have strongly enforced incentive to be more dedicated to their goddess than most societies (and thus more strongly aligned on average).
