I seriously think that the Aztec lack some kind of combat bonus which last longer throughout the game. Lets have a look on the other typical warmonger:
- Assyria: XP bonus from GW
- Denmark: Viking Fury and Viking/Longboat Promotion
- France: +10 % CS vs already attacked unit/city
- Greece: + 5 % CS up to +25 % CS for every City State Alliance
- Japan: Bushido Promotion for melee Units from Dojo
- Mongolia: +2 Movement and ignore ZOC for ranged mounted units
- Persia: +15 % CS and +1 Movement during GA
- Songhai: War Canoe + Amphibious Promotion, extra Movement along rivers
- Sweden: +20 % CS for attacking melee units, +1 movement for siege weapons
- Huns: more flanking damage for mounted Units (+ capturing units)
- Iroquois: road movement on forests/jungles and woodsman promotion
- Zulu: melee/gun units cost less maintenance, less experience for promotions, buffalo promotions for melee units from ikanda
Only Rome is comparable, usually a warmonger too, gains sth from conquest, but only has additional GG/GA points from kills, so somehow like Portugal (with extra GG and GA). The only other civilization not mentioned yet who gains sth from wars is china for conquering cities. But unlike those mentioned warmongers above, those civs have boni not directly related to war.
The Aztecs UA can only benefit from wars, a peacefull Aztec just has his floating gardens. The only fighting advantage I can see for the Aztecs later in the game is, that they will have extra faith and gold to purchase units with Zealotry, buying directly (which got nerfed with just half the xp) and better upgrading.
(Yes I know, the iroquois are not that usual warmonger)
I agree that the Aztecs time to shine is the early game to take out one or two neighbours. But then it is the question, you want to stay in something like a permanent war killing units left and right. But to trigger the golden age, you need to conquer cities (and raze them) or you have to pillage your enemy to oblivion.
My problem is just, they lose any advantage beginning with medieval era and if the Aztecs didnt gain a big advantage until that point, they will likely fall behind. In my last games, the Aztec AI didnt perform well. It often stood for a long time with just one city and I have the feeling that the AI gets to much distracted because of hunting barbarians all around the map with its UU.
But how to buff the Aztecs? I dont think they need additional yields from killing. Giving healing to melee units might be sth. Or a slight combat bonus, maybe situational like other. But I highly support any combat bonus for the Aztec which will last throughout the game...
EDIT:
I very like the Idea of
@ElliotS to give the Aztec a bonus against wounded units. But I would suggest only a little one, because that kind of bonus can become crazy strong in a good combination of ranged and melee. Maybe only give melee units a combat bonus to wounded.
And if they kill it, give a bit of health back.
That packed as a unique promotion given to units build in a city with floating gardens or maybe the temple.
Might make the original jaguars too strong, so maybe they loose their healing promotion after upgrade and get it back in a nerfed version with the promotion but with enhanced fighting strength. In meantime if you upgrade your units, they can benefit a while on Discipline alone, so the power of the Aztecs is a bit weaker after Jaguars and comes back with Promotión on floating gardens/temples.