There are 3 ways to regenerate your Soul Points:
The first one, as you mentioned, is killing creatures. You need to kill a creature which experience is equal or higher than your level. This way, if you're a promoted character, you will regenerate 1 soul point each 15 seconds (or 4 per minute). If not, you'll regenerate 1 soul point each 2 minutes.
Second one, is sleeping. You'll regenerate 1 soul point each 15 minutes.
Third one, standing in resting areas. After you reach 7 days streak of claiming your daily rewards, you'll be able to regenerate soul points by standing in resting areas. This works like when you kill a creature. If you're promoted, you'll regenerate 1 soul point each 15 seconds, if not, it's 1 soul point each 2 minutes.