The race is led by a powerful celestial-king who enforce a supernatural law that will make a member expelled from their land/world/dimension if said member do something that's not 100% Good. Naturally, most people choose the path of Good for their own convenience.
Alternately, the race is from a Death World where they must band together to survive, and thus uphold high moral standard to keep things from falling apart.
Alternately, they were artificially created to be Good/so zealously altruistic that they're, by any standard, Good.
Alternately, they are part of a Hive Mind which is dedicated to Good.
Alternately, is needed be Good to turn yourself into a member of the race.
Alternately, their gods simply made them that way.
Similarly to how the Fey cannot understand lying, the race doesn't understand cruelty/murder/etc. and as such no one of said race has those qualities.
Characters find a Good angel, but it wants to kill them and take their belonging.
The race isn't Good, but it is portrayed as such as a massive propaganda campaign by said race.
The 'Good angel race' turns out to not be entirely Good, since some are on the evil side, until the innate spark of Good-ness in their blood cause the evil angels to switch sides when the world start to fall apart.
After all those homicides and robberies the characters do, it would only be right for an angel to kill the characters and bring back the belongings.
The propaganda campaign is carried out via word of mouth by people who see and experience the constant good deeds of the Good Race. Just because they're kind and altruistic doesn't mean they don't like tooting their own horns.
Parodied: An angel who is easily fooled/manipulated/bullied into doing Good things heroes are supposed to do, because, you know, angels are supposed to be Good.
Zig Zagged: The angels are seen performing apparently-evil actions against the heroes. The heroes start to view them as evil, until it turns out those evil actions really were necessary, and Good Is Not Nice after all. However, having got a taste of nastiness, some of the angels decide they like it and start doing unnecessary villainous acts For the Evulz. The remaining good angels confront them with a What the Hell, Hero? type speech, and the evil ones all simultaneously go into Villainous BSOD at the Heel Realization that they've become the bad guys.
Averted: There is no 'good angel race'. Good is made up of races that are also just as likely to be evil.
Enforced: The race come from Heaven itself- if we portrayed them as being unfriendly in any way on a popular show, the Moral Guardians will not be happy with us.
Lampshaded: The heroes ask a member of the 'good angel race' why she's helping them, and she answers with "Haven't you seen? My whole race is Good!"
A member of the race begins preaching that they all have free will and do not have to simply be Good. This gains credibility among everyone, some members turn Evil, some remain Good. The ones who remain Good feel reinvigorated because they have now chosen Good, and become more Goody-Good than ever. The Good members proceed to kill theEvil Overlord, because lolGood!.
"See, if this were a fantasy book, I wouldn't be able to beat the living crap out of you and take your belonging guys, since angels have to be Good and all. You guys are just plain unlucky!"
"I'm an angel; smiting evil and saving good is in my blood."
1. The angel race are all Good... because they discarded their free wills and subjected themselves to Hive Mind dedicated to Good-ness. Eventually the Hive Mind is dismantled (for good), and they have a hard time deciding just what is Good on their own.
2. The presence of an Always Lawful Good race leads to discussions of moral relativism vs. moral absolutism and whether or not we can force our moral standards on a race.
3. Everyone assumes that a celestial race is all good... thus forcing an utopian sense of morality to celestials who, despite their great abilities, has human limitations.
4. Members of this race are prone to stress, insecurity, unjustified guilt, and messiah complexes. Their very nature drives them to be heroic in their every waking moment, but nobody's perfect. Whatever drives them to this altruism doesn't have an off switch, and they just can't manage to forgive their own failures and shortcomings, not when innocent lives may be at stake.
2. They cut the knot by stopping the discussion and deciding to be nice to each other. They help without being overly preachy.
3. The celestial race, as a whole, does their best to live up to expectations, and though they're not perfect, they're an example for everyone else.
4. While good, the members of the race recognize that they are imperfect beings, and have learned not to beat themselves up over every little mistake. You can't change the past, so the best you can do is admit you were wrong and try to do better next time.
5. The race is a Proud Warrior Race Culture of Martial Pacifists. They do their best to look for the good in others and give them the benefit of the doubt, but if they are attacked they respond in kind. They had a sister race with good and evil impulses but it bred so many Blood Knights they caused themselves more trouble than invaders.
6. Their normal is our good and we don't like Smug Snakes so they abhor them. They do their best to be Humble Heroes and say Think Nothing of It. Nonetheless they are accused by other races of thinking themselves better than them because they are so humble.