David Conway argues that the proposition "evil is omni-preventable" is true, regardless of whether it is necessarily or contingently true. He chides those who might make such modal distinctions regarding truth as necessarily or contingently true (at least, in this case he does). For a truth to be necessarily true means that it's true in all possible worlds. Contingent truths are true in some possible worlds, but could be false in others.
Let's accept Conway's claim that the proposition "evil is omni-preventable" is true although I have reservations about his approach since the modal distinctions make a huge difference here. I do not believe that it will suffice to say that the aforegoing utterance is true. Why is that the case? Imagine S (a 3 pack a day smoker) who has made a choice which could be described as "evil" (morally speaking) which leads to yet another evil, namely, lung cancer. Granted, one might argue that God could have prevented S from smoking 3 packs a day and he could have prevented S from getting lung cancer by dint of this "evil" habit. Yet, in order for God (the omnipotent and omniscient being of Christianity and Judaism, etc) to prevent these two evils, it seems that God could not have created S with the ability to perform (or refrain from performing) the action (A) which brought it about that S suffered the evil of lung cancer.
Even if Conway wants to argue that God could have made the world such that S could never have made the decision to smoke, etc, I truly do not see how that God's not preventing the aforementioned evils work to disprove his existence.
In summary, one of my objections to Conway's claim regarding the omni-preventability of evil is that he has failed to flesh out what he means by "evil" (moral or natural). It is not sufficient to say that the proposition "evil is omni-preventable" is true, even if one accepts Conway's point about the modal distinctions. We now have to make subtle distinctions regarding evil and show why God should bring it about that S cannot choose to smoke 3 packs a day, drink alcohol in great quantities or drive 120 mph on I-40, all actions which could be characterized as "evil" insofar as they have the potential to wreak great harm.