Always an interesting topic. For myself I see good and bad (don't like the word evil, too many religious/supernatural connotations in my head) as general constructs useful for society that are to a large degree relative to the society/culture someone is in. Calling good "beneficial" and bad "harmful" also works. Could say that is semantics, although less baggage with those words than good/bad.
What I don't know myself, but feel that it might be true (while at the same time knowing what I feel is heavily influenced by who I am and my past experiences), is whether there are some general universal validity to a concept of good and bad, that crosses culture/society. Not specifics but ideas that are shared in other members of the animal kingdom as well, that in general harming another is bad and helping another is good. Of course behavior is also extremely situational, but you can't define good/bad as absolutes in my opinion.
That doesn't make them useless though. They are important for every-day society. Much like in physics when you learn that chair you are sitting on isn't really solid, its mostly empty space. For every day purposes it is more useful to consider it solid.
That is my mindset anyhow. I try to keep an open mind and remember good/bad is both relative and learned but at the same time I think the concepts and words still are useful and important to society and interacting with others.
Another issue is behavior is also governed by what you can get away with. When their are zero consequences, and no one else knows, people are more likely to resort to purely selfish behavior.
Well too many tangents and a long topic. So while aware of how relative, situational, and vague the concept of good/bad is I still find it useful myself to help label and define my own behavior as well as others.
So when adapting to a game world I might ponder the philosophical discussions I still enjoy dealing with the more concrete issue at hand.
So True Neutral - that tends to be everyday animals in many definitions since they don't have a learned sense of morals imposed on their behavior like humans do. They mainly follow their instincts and survival needs. Although some studies on certain animals suggest some have a low level concept of acceptable or not-acceptable behavior … but another very large topic with many disputes. Gamewise I see TN as those who seek balance and remain impartial to concepts of law/chaos and good/bad. Consequently I find it a very difficult alignment to play because being that impartial is difficult. It is the most demanding as you try to weight all consequences and strive to act at some deeper philosophical level, while chosing your actions based what you hope is logical and not emotional.
Even there I have some issues with it. What's the purpose? Is it in some ways a "good" purpose in the sense that keeping everything in harmony and balance is ultimately good for everyone and everything?
I admit I don't fully understand its goals beyond some sense of balance of all forces and not letting emotion sway decisions. Maybe why I avoid it as I am not sure how to "play" it.
LN I also don't fully get. You are neutral in your choices but only as long as they don't break any laws? I suppose that could work as a definition. I also see it as a colder alignment, like the Vulcans from Star Trek.
CN is what I see as more human out of all the alignments. TN to me means no learned concepts of good/bad or law/chaos getting in the way, more the natural world and its animals. Or for humans TN focuses on controlling your actions so you keep everything in balance/harmony. But CN means just doing what you want (as long as you feel you can away with it). Impulsive, acting mostly on your emotions. You can still be smart and logical about things but you lack structure and care nothing for laws. It means good/evil is much more irrelevant as long as you don't get caught or punished.
Lastly for good/bad axis I return to my start by saying the terms are useful for day to day and hence use my own concepts of those terms, which often align with that of most people I find - the developers, people in other countries I chat with, and so on. Getting into specifics or grey areas start to mean more disagreement on whether something is good/bad but that, to me anyhow, doesn't make them pointless as I still find common ground with most people I have met in life on the idea that hurting others is bad and helping others is good.
EDIT: Forgot to comment that I also like Skyrim/FO for the reason you make your own morality versus the developers overall. Meaning I have to control my own actions (impose my own morals) on doing things. I tend to like that as it provides a lot of freedom.
That being said I can also fully enjoy a game like PF:KM where some of that is more imposed because you have to make choices with consequences in the game itself. I also find that very enjoyable .. how much so I suppose depends on how well I agree with the overall design.
I certainly didn't always agree with the developers alignment choices and thought some were way off ... but that was fine. For me the fun is watching my alignment shift around, seeing how choices play out (love that) and so on. But for pure RP freedom find Skyrim and FO a bit better. Both are good for me though.