The underlying AD&D system uses a complicated set of rules (most determined by statistical probabilities originally based mostly on rolling polyhedral dice, which change as players level up from gaining experience points), and requires players to generate a player character before starting the quest.
You can choose from a wide range of fantasy races, classes (fighter, mage, thief, cleric, etc), allocate attribute points (strength, dexterity, intelligence, charisma, etc), and special skills that go with particular classes.
This type of flexibility could lend itself to a good pacifist run in theory— though in practice a lot of computer RPGs that use such systems still tend to make it impossible to avoid mandatory combat in lots of scripted situations. Fortunately, Baldur's Gate I held up quite well for a long and enjoyable pacifist challenge, with only around three key adversaries (all "evil" alignment key villains) that the game forces you to "dispatch" in some way, with no other option.
For the purist, the first two villains could optionally be dealt with by casting "charm" enchantment spells on their own killer minions to go fight with them, which not only "gets the job done" where there is literally no other option, but avoids rewarding the player with any experience points for causing a death.
On the even more positive side, the game is full of fun nonviolent skills like the thief's hide-in-shadows stealth skill, invisibility potions, and many inventive mage spells like "color spray" and "sleep" (to knock enemies unconscious without harming them), the essential "charm" spell (which bewitches adversaries for a short period of time to make them join your side), and the invisibility spell.
In AD&D tradition, the game is also full of treasures, including gold and gems, but also valuable magical relics to aid nonviolent adventurers: Magic books granting abilities, wizard's staves that allow wielders to cast spells that paralyze or enchant enemies into a temporary sleep without even injuring them, cloaks that allow one to remain undetected by hostile wizards, and even a cloak that allows you to transform into a wolf in order to run away from foes at a higher rate of speed.
Back when Baldur's Gate was developed, the "pacifist run" and the phenomenon of players wanting to complete RPGs without killing anything was pretty uncommon, so being forced to bump off "only" three evil villains by making their own minions attack them is pretty good, considering there are hundreds of enemies and dozens of encounters throughout the game that can be dealt with using stealth, enchantments, or negotiations without having to kill anything.