At one point she asks Kazuma if he does in fact want power, and he (initially) turns it down. We don't actually see his flashbacks in full, at least in the anime, but we see two major images of her. Him swearing to protect her (which later is the question Ayano asked him on why he signed the contract, under Armor-Piercing Question), and her getting fed to a youma. His eyes turn to red when he's possessed by evil, but if it were just anger fueling his power, it shouldn't be changing his power, or his eyes. Now, on the other hand, if the Spirit King that was the source of his power was tainted by evil, it would explain how Kazuma might be having problems like this when his heart is weak.
Ren is destined to gain wind and earth powers
Ren's big arc has him saving and protecting a girl with sufficient earth power to seal away a living embodiment of a natural disaster, whose body was built around the egg of a pixie chief. Given the fact that magical beings had been shown in other parts granting powers to individuals for various reasons, it wouldn't be hard to imagine a future arc in which Ren woke up to see a pixie gazing at him, or for said pixie to grant him powers. And since the pixie chieftain in a sense lived briefly as an earth mage, this could easily translate further to him being given both earth and wind powers. If the author hadn't passed away, after a year or two the story could have had a young boy simultaneously wielding three elements in battle.