Okay. I'm not saying my answer is perfect. I'm just saying it's my answer.
Is God willing to prevent evil, but not able?
Is he able but not willing?
Is he able and willing?
Is he neither able nor willing?
The answer is God is able to do all things. Is he willing? God's will is higher than everyone's will. We humans do not understand all reasons for everything evil or good that happens. Some things we question and sometimes, they make us question our faith and the existence of God.
If you look at Genesis, God created man and gave him free will to do what he wants to do. If humans didn't have free will, then we would all be robots which is not what God wanted. If he was willing to stop man from making evil choices, robbing, killing, etc., then God would not have created a being with free will as He intended.
Cain killed Abel. His brother. Cain had total free will. It's kind of strange, but it shows the depth of the free will given to man. We are not robots.
Epicurus is questioning why God doesn't intervene evil from happening. And because evil happens and God doesn't intervene, he believes that God doesn't exist.
Why didn't God stop Cain? If God stopped Cain, then He could have. But it was not in His will. Daniel was thrown into a den of lions, but it was God's will to save him. Why? It was God's will.
Abel was innocent. Cain was the bad guy. Abel didn't deserve death. What good came of that? It shows that God truly created human beings with total free will. Proof of God's ability. Kind of dark. If everybody was a goody-two-shoes, we would all question, did we really get free will as He originally intended?