You hit on a logical truth.  It makes sense.  This thought experiment helped me, so take it for what it’s worth.  Every system needs an objective standard that exists outside that system against which to measure things.  The thermometer or the ruler exist outside the experiment, they have to in order to measure anything.  We cannot say anything is good or evil without an objective standard that exists outside of ourselves, exists beyond humanity.  You cannot make a value judgement on things that are better or worse, “gooder or eviler.”  Otherwise, logically, you are only making a subjective measurement that is just as valid as the one calling the evil thing “good.”  Without God defining good or evil, we become the arbiter of good and evil and thus try to make ourselves God.  God must be outside of Creation, not a part of all that is, in order to be objective and not defined by the Creation itself.  In order for you to call a truly evil thing, evil, which we know exists, there must be an objective good against which to define that evil thing.