The level difference between you and your opponent will directly affect the guild experience you receive:

Fighting enemies who have a higher level than you will yield more guild experience.

Fighting enemies who have a lower level than you will yield less guild experience.

Note: In practice, this just means you should fight the biggest enemies you can safely take.

While a playerkiller, you will receive a global 25% bonus to all guild experience earned. This bonus is applied after the level-differential bonus/malus as explained above.

Your guild experience gains will be subjected to increasing penalties once you pass guild level 500. Sorry, that's the price of success.