Dungeons and Dragons PnP: Do Angels have to be good and Demons have to be evil?

They don't care for balance...so why would they team? Demons are creatures of evil and chaos after all...

There are always exceptions, especially in the current version of the rules, where alignment isn't as heavily emphasized. But even in earlier editions, alignments for a given intelligent monster were just what could be expected from an average specimen...individuals could always be found that, for whatever reason, strayed from that norm. (Unintelligent types were another story, since it generally required conscious choices to break from the norm.)

They don't care for balance...so why would they team? Demons are creatures of evil and chaos after all...

Not necessarily - Angels refer to the servants of a deity. Thus, angels of an evil god are evil. Demons, however, don't always have a god, but follow evil kings and masters. They have the potential of escaping their owners and striking out on their own. Really, they could be both!

Not necessarily - Angels refer to the servants of a deity. Thus, angels of an evil god are evil. Demons, however, don't always have a god, but follow evil kings and masters. They have the potential of escaping their owners and striking out on their own. Really, they could be both!

Depends on the setting. In most cases its possible, but incredibly unlikely. Demons can seek redemption and angels can fall, but this is generally a once in a millennium event.