Its not the killing and Not the treasure that give people experience. Someone doesn't just transform incrementally by every creature or person they kill in a few minutes of combat. In normal lives it is the time in work we do that transforms anyone.
Time and Work - An adventure has in it elements of time and work. Success is just objective proof and validation of being in the right. Theoretically someone with a near omniscience level of self awareness can pretty much figure where he went wrong and with enough work correct that in himself and train to be better...theoretically.
Imagine what happens in the real world adventure: a military campaign. Soldiers train and train, and only the lucky and the practiced survive to improve and reap the rewards.
I was writing my GM section when I had to boil down my ideas of what is Experience. In making a home brew or adjusting my GM style to fit the reality more so that the consequences that follow are more believable and natural, I thought the xp system should reflect this fact.
When telling another GM how to reward players, there is a whole lot of game theory going on in any system of rewards. In particular, the Philosophical aspect of Growing and Changing.
Does Killing People build Character, how about marching through rough terrain, starving while waiting for reinforcement, near-TPK in "in-character" disagreements etc...? Trials, Pain, Suffering, Loss and Hardship is the physical stimulus that signal change is needed and is going on.
As for Levels of how much XP to give, imagine how much change a soldier goes through in his first campaign. Consider the kind of learnings, memories and experiences he comes away with. Then proceed to consider how many more of the same, before the next bit of change, then the next and the next.
A rule of thumb is that, experiences have diminishing returns. The First campaign is enough to level up to 2. In the next campaigns, if similar to the first, might take a few more to level 3. If the character escalates in responsibility, accountability, and magnitude of effect (rising through the ranks) it may take just a couple for level 3. Over time, escalation of challenge plateau. The character becomes one of the best generals or warriors, and being any better may half another life time. In fantasy games, plateau are less likely as characters become epic, demi-gods and gods.
In more low tech and realistic settings, at this point they diversify: they become conquerors, which requires politics, administration, diplomacy, espionage etc.
Diminishing returns are important for the GM to reflect. If its the same-old same-old then there aren't any "unknowns" being explored, Gaps in the abilities of the character. then the are not gaining much or anything new.