The first one is definitely a hybrid. It had lots of adventure elements (including some rather clever puzzles), and was in fact rather slow-paced most of the time. Quite a few "action sequences" were actually standard adventure puzzles, just with a little tension and timing (which is also something that existed in other adventure games at the time); it was sometimes possible to solve a puzzle to avoid a fight (in the dining room, for instance), and some sequences were just timed puzzles (the library). So there were many adventure elements.
Still, some of the action sequences went far beyond just adding a little tension; many of them were unavoidable, and could be very challenging and relying solely on dexterity (such as the sword fight with the pirate, or the underground sequences).
So I'd say the first one really was a hybrid (and great game too), which required the player to deal with elements of both genres, but often let him/her free to play the game more as an action game or more as an adventure game, depending on his/her preferences.
My memories of AitD 2 and 3 are much fuzzier, but I remembered them as not having really found the same balance, being much more action-oriented.
EDIT: Oh, and JemyM: If you think AitD was the first adventure game that allowed you to fight enemies, you need to play more old games.
