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
It was pretty clear the implication that combat was pretty strongly expected.
1e DMG P 85:
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:
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
IOW, all that stuff that isn't killing and looting is "conducive to non-game boredom".