On 107a, David Hamelech says that he could have overcome his yetzer hara but didn't want to so as not to show that HKB"H was "wrong" in saying he couldn't. How does this fit with his asking for a nisayon, or what the purpose of a nisayon is?
Gershon
The Margaliyos ha'Yam reads the Gemara differently. He explains that David Hamelech was saying that if Hash-m would not have taken away His usual help which he gives people who have free will to make the right choice, Hash-m knows that he (David) would never have sinned. It is only because Hash-m took this away that he sinned, in order that people should not say the servant triumphed over his Master. [According to this explanation, when the Gemara says "Ela Amina," it means that David is saying the reason why Hash-m made this so, not why he did what he did.]
All the best,
Yaakov Montrose