In the context of this argument, that is an inference to what God must be like to be the ground of objective moral values, but it has independent justification in some of the more comprehensive forms of the ontological argument. Similarly for the obedience due to God. (The justification via the ontological argument depends on the premise that necessary goodness and worthiness of worship and obedience are forms of perfection, and I admit can only offer intuition in support of those premises. Nevertheless, I think they are more probable than not.)
But we do have more than āwhat God is likeā; we have the moral intuitions that God has given us to guide us in what is good. These are fallible (because of our fallen nature) but still a valid source of knowledge.
Thereās obviously more than one such difficulty of the kind you reference here, and the response may vary depending on which one it is, specifically. I recommend Paul Copanās Is God a Moral Monster? for its treatment of such issues. There was also a thread on the forums here a while back that had an article on the interpretation of the book of Joshua, in the form of a dialogue, which offers an alternative to Copan on the Canaanite genocide question specifically. Clay Jonesā Why Does God Allow Evil? touches on your objection here as well.