Only specific kinds of alignment questions are on topic.
There are a variety of kinds of alignment questions, some of which can be constructively handled by our site and some that can not. There's not really a need for special rules for alignment questions; these are covered by the usual close rules (too broad, opinion based, etc.) but it can be hard to see how these apply in certain cases.
Note that some "is this good or what" alignment questions on the site have not been closed - but they have been identified as XY questions and answered outside their frame. Like top voted answer on Good alignment on characters facing a vampire leads off with explaining what "Always X" really means, and a good answer to the first linked example in the question explains "one act doesn't change your alignment."
The trouble with alignment questions is that the alignment rules for the most part are deliberately vague about what exactly is good, evil, lawful, or chaotic - they provide a brief description but then leave it to you to fill in your own RL moral code in most circumstances. And people's moral codes are not good topics in the Stack Exchange format. You'll note that even the religion sites (Christianity.SE, Mi Yodeya) have had to resort to saying "you have to cite the specific viewpoint of a denomination/acknowledged authority" to avoid this problem.
The Good Kind
So it's appropriate and on topic to ask alignment questions that are:
- Mechanical - how does changing alignments work, etc.
- Social - people in my group don't like me playing an evil character, etc.
The Bad Kind
But the kind of question that is always "primarily opinion-based" is "what alignment is act X/situation X/character X/society X?" Except for the couple very narrow exceptions (e.g. in D&D 3.5e, casting an evil descriptor spell is explicitly defined as an evil act) this is basically asking "what is your philosophy of life," which is not a good topic and is impossible to answer even using Good Subjective, Bad Subjective - you can say "I ruled that as evil in my game" and that's experience, but it's unhelpful and irrelevant. The SE format is not intended for or appropriate to topics that basically just become popularity voting on completely subjective opinions.
The right answer to all of these questions ends up being the same, which is to turn the specific question into a process question of "how do you figure this out yourself," and a review of all these questions on the site shows that the meaningful answers all end up composed of one or more of these points:
- Alignment is subjective and it's a DM call
- Alignment should be descriptive and not prescriptive
- One act usually shouldn't change your alignment, depending on the system there may be optional rules that have been introduced to model alignment shift (here's Pathfinder's)
- Here's a bunch of descriptions of good and evil and lawful and chaotic stuff from the core rules and various splatbooks you can use to guide your decision
- Various general warnings about social issues (it's debatable, it can cause player dissent, chaotic and/or evil and/or lawful good characters could make the other players frustrated, etc.)
- Alignment is a dumb horrible system
But in the end it's the same answer for all of them. Just point them here. We should close these and change the tag wiki for alignment to say "Alignment debates over what alignment a person, act, or society should be are off topic."