That’s why I prefer to distinguish between a character’s ideology and their actions. The above character sounds like he holds Good ideals, but commits Evil actions. Which is fairly typical, to be honest. Most people tend to hold altruistic ideals in theory, but in actual practice behave more egoistically. At my table, that character would have Good written on his character sheet, but would count as Evil for the purposes of any mechanics that care about alignment. But, of course, ignoring alignment completely works just as well. Better for 5e, in fact, since 5e has almost no mechanics that care about alignment anyway.
I would imagine that the majority of people do not know their own alignment, and misrepresent their alignment to others and even to themselves. In D&D terms, it requires a high degree of Wisdom to be self-aware enough to know what you actually believe and what it means. This is complicated by the fact that no mortal however wise is going to be absolutely pure in their alignment, but will depart from it consciously or unconsciously at times in a variety of ways. And again, in D&D terms, the lower the character's Wisdom, the less understanding and willpower they will have to actually perform the deeds that they claim to believe in.
In the case of Maxperson's hypothetical child abuser, I don't think many members of the community will - once they discover his secret vice - think twice about labeling the man an evil monster. All of his public good deeds are actually rather small acts which barely rise above the level of neutrality. They create a few small moments of weal and health, compared to a potential lifetime of pain, suffering, and mental unhealthy in the man's victims. The two things aren't even comparable. They are clearly a public show that does not reflect the man's real character, which is better reflected in his private acts. We in the real world recently had a series of scandals in which public figures who had squeaky clean public images, or which had received awards for their meritorious public statements regarding women, in reality behaved in this way in order to create a plausible front behind which they could safely engage in abusive and nefarious behavior.
I find it strange that someone would speculate that a person regularly abuses children to satisfy his appetites might possibly be good, or even just good on the net. I mean, in certain sense I agree that people are more complicated than alignment, but I'm baffled that anyone would think that example demonstrates that or provides some sort of strong objection to having alignment in a fantasy game.
I tend to prefer in my games to have what is on the sheet reflect the characters actual alignment, and trust my players to play character's in a nuanced and complicated fashion. Of course, they don't always do that, but at least then the player has no excuse for not knowing how I judge the character's actions. So, at my table, the child abuser would certainly be evil of some sort and the player would be expected to know that, regardless of what the character believed about himself. The character can believe that he is Lawful Good, and that he has a right to perform the abuse, and that it's for the best in some fashion or whatever other monstrous justification he gives himself. I'd be very uncomfortable with a player acting out a character in a monstrous fashion of any sort, and not realizing that the character was a monster. Heck, when it comes to a PC actually being a sexual abuser of children, I think I'd probably have to put a stop to that OOC because that would be some seriously sick crap.
I can only report on my experience with groups and players. And my experience suggests to me that at least the plurality of groups and players that have a problem slapping an alignment label on their character sheet, have that problem precisely because they want to think of their characters as being heroic and good, even as those characters engage in a series of monstrous acts - cold blooded murder, torture, mind rape, arson, theft, etc. They tend to play characters that like Maxperson's hypothetical child abuser, believe that the little bit of good they are doing, or the good cause they claim they are advancing, outweighs the fact that they are behaving more or less indistinguishable from the villains aside from - maybe - the color of the hat that they are wearing. Every deception, every murder, every theft, and every act of abuse is justified because they need to win. When I get those players at my table, I tend to just encourage them to play Neutral characters, and explain to them that most people in the campaign world don't believe alignment is all that important, or that it is entirely situational, or that ultimately what is right is what is practical and utilitarian. It's a harsh world and they are just trying to survive and don't have time for philosophical arguments.
This tends to make both of us happy. If it turns out that they really want to play an evil character, they just didn't want to think of the character that they played as evil, I find I can usually bribe them into putting evil on their character sheet, by promising them some token reward like 100 XP "good role-playing" reward, if upon performing an evil act, they'll adjust Nuetral or Chaotic Neutral to Neutral Evil or Chaotic Evil. And again, this makes both of us happy. I now have the correct alignment written on the character sheet, and the player feels like they are 'winning'. Win, win.
