Like a lot of things AD&D, it was pretty schizophrenic.
For example, while you can talk about xp for "tricking" monsters being in the 1e DMG, you also have the training rules. A fighter that didn't fight was actively penalized by being forced to take longer to train and spending far, far more money on training, for example. In 2e, while there were "bonus Xp tables" again, fighters ONLY gained bonus xp for killing stuff.
Add to that the published modules of the day, which again, leads to a VERY schizophrenic experience of 1e where the DMG advocates one thing and the modules pretty much entirely ignore the DMG, and it's very easy to see why murderhobo play was pretty common.
Referenced: 1e DMG p 86
Clerics who refuse to help and heal or do not remain faithful to their deity, fighters who hang back from combat or attempt to steal, or fail to boldly lead, magic-users who seek to engage in melee or ignore magic items they could employ in crucial situations, thieves who boldly engage in frontal attacks or refrain from acquisition of an extra bit of treasure when the opportunity presents itself, "cautious" characters who do not pull their own weight - these are all clear examples of a POOR rating.
It was pretty clear the implication that combat was pretty strongly expected.
1e DMG P 85:
The judgment factor is inescapable with respect to weighting experience for the points gained from slaying monsters and/or gaining treasure. You must weigh the level of challenge - be it thinking or fighting - versus the level of experience of the player character(s) who gained it.
And then there are pretty complex maths used for calculating that xp. For stuff that isn't killing and/or looting, we get this piece of advice:
Tricking or outwitting monsters or overcoming tricks and/or traps placed to guard treasure must be determined subjectively, with level of experience balanced against the degree of difficulty you assign to the gaining of the treasure.
IOW, if you kill the monster and take the treasure, you are guaranteed a certain xp award. If you trick the monster and steal the treasure, your xp reward will be based entirely on whatever you DM feels like. You tricked them too easily? Oops, sorry, no xp for you. And, frankly, that sort of thing just leads to far too many arguments at the table. So, DM's and players both shied away from it and relied on the codified rules.
And, lastly, we're left with this bit of advice on page 85
Note: Players who bolk at equating gold pieces to experience points should be gently but firmly reminded that in a game certain compromises must be made. While it is more "realistic" for clerics to study holy writings, pray, chant, practice self-discipline, etc. to gain experience, it would not make a playable game roll along. Similarly, fighters should be exercising, riding, smiting pelts, tilting at the lists, and engaging in weapons practice of various sorts to gain real expertise (experience); magic-users should be deciphering old scrolls, searching ancient tomes, experimenting alchemically, and so forth; while thieves should spend their off-hours honing their skills, "casing" various buildings, watching potential victims, and carefully planning their next "job". All very realistic but conducive to non-game boredom!
IOW, all that stuff that isn't killing and looting is "conducive to non-game boredom".