I'm a huge fan of watching the AI fight each other, whether that's spawning demons in Dwarf Fortress's Arena mode or carting around a suicidally incompetent security guard in Half-Life. Enter CacoFrendo, a mod for the original Doom that lets you pet the Cacodemon until he becomes your pal and fights on your side.
"does ur heart ache every time u squeeze the trigger on one of the lovable tomato boyz?" reads the description on the ModDB page. "well now with the power of 'Cacofrendo' you dont have to! simply pat the Caco on his lovely little head and watch the blossoming friendship take hold!"
You can see the pat-to-befriend method in action in this tweet by the mod's creator:
There are probably two distinct categories of AI fights I enjoy. One is where you crest a hill and stumble upon a fight between two enemies. Perhaps they're also your enemies, but they're fighting one another for their own purposes. There's little in games I find so evocative, or which renders a world so immediately convincing, as an unscripted rumble between NPCs. It's the whole reason I like Stalker - and a joy on which Jim once waxed at length.
The other category is the Half-Life security guard, the Brogue monkey, and now the Doom Cacodemon. Escort missions are clearly rubbish, but make it optional and systemic and suddenly I'll go through enormous pain to keep an idiot NPC alive. This is the joy on which Alice O wrote more recently in 'What's Better?'. (Ultimately deciding that air control was better).
If you also want to try fruitlessly to keep a Cacodemon alive and then feel sorta bad when you fail, then check out Frendocaco on ModDB.