They are Evil (Lawful or Chaotic) in AD&D/AD&D 2e and "Always lawful evil"/"Always chaotic evil" in 3e/3.5e. Starting with 4e, D&D started to go soft on monster Alignment in general.
One of the reasons 4e didn't do so well perhaps?
The game is Dungeons and Dragons. The epitome of final bosses, the highlight of every quest, is to fight an evil dragon. As Volo said, "Every story benefits from having a dragon."
