We Never Know
No Slack
If evil is subjective, how can good, and therefore omnibenelovence, also not be ?
And if good and evil are a matter of opinion, then I could claim God is omnimalevolent and no one could say that I am wrong per se, only that they feel differently. That's a pill hard to swallow for many theists for two reasons: The first one being that they believe in objective morality and the second one is that they want to state that it is a matter of fact, not of opinion, that God is omnibenevolent.
Other than that, even if evil is subjective, many people still feel the need to justify the evil they see in the world. For instance, I would guess that it would be disconcerting for most people to say (and believe) that children dying from smallpox was an act of omnibenevolence per se. So, depending on how people use the terms 'evil' and 'good', the argument is still applicable.
Do you think a rabbit that had babies and one dies of a disease thinks its evil or does the mother rabbit and the rest of the babies simply go on to try to survive? Point being, evil doesn't exist to them.