I would say one of the burdens of being "Good" is realizing you can never really MAKE that sort of qualifier, and having to take each case as they come as individuals. They often say "Evil is easy" and I think that's a good example of how - it's easy to just lay down a rule of "All beings of type X are Y morality, always do bad things even if you don't know about it, and automatically deserve punishment without investigation" but it isn't right to. Good has to actually care about the acts and history of an individual before judging them (and should care about the circumstances and reasons that individual ended up as they did and why, even if it can't change this specific judgement), evil doesn't have to bother with all that hard work and can just slap a broad rule down.
The cosmic forces of D&D are definitely wonkier than the mortal races, but there's certainly existing, if rare story instances of demons (less often fiends) who haven't really done anything bad, even if they're not trustworthy. Or petty kinds of bad that don't justify ultimate destruction. It's definitely trickier ground because of the whole divine aspect... but I'd personally say morality wise, when you can't be sure, you have to err on the side of caution and mercy if you want to be "good" yourself.