You can accept the extension of the category "evil" to be whatever theists want. It appears to follow from essential theistic claims--that God, is the omnipotent, omniscient, creator--that evils exist only if permitted by God. It also appears to follow from the further tenet that God is perfectly good, that he will permit an evil only if he has a morally sufficient reason for permitting it. Consequently, every actual evil, i.e. every evil that God permits, is such that God has a morally sufficient reason for permitting it. But what possible grounds could anyone have for holding that for every actual evil E God has a morally sufficient reason for permitting E? No theodicist claims to be able to account for every evil, but only to give some justification for broad categories of evil. Yet a single actual instance of gratuitous evil, i.e. evil for which God would not have a morally sufficient reason, is incompatible with the existence of God. Theists must therefore have some basis for confidence that there is no gratuitous evil. What could that basis be? I suspect (sigh) that, in the end (sigh) it will come down once again to the "F" word: Faith.
Several basic problems:
i) Atheists often cite examples which they deem to be evil. For example, they cite divine commands or divine actions in Scripture that they (the atheist) deem to be evil. So that's not defining evil to be whatever theists deem to be evil.
ii) Aren't there theists like Peter van Inwagen and Mike Almeida who think gratuitous evil is compatible with God's existence?
iii) It's misleading to suggest that every single instance of evil requires a separate explanation, for every evil ultimately has a single ultimate cause in the will of God. So don't we just need a general explanation for our belief in God's goodness and wisdom? Indeed, isn't that in fact premised in the argument from evil ("God is perfectly good").