Then good and evil have no meaning.
If you cannot say "God would be evil if He did this," then you are unable to say "God is good".
(Allow me to defend the usage, or at least the ancient usage, of the word "good" or "benevolent" in describing God)
God, in traditional Christian view, sustains everything that exists until now (yes,
even the devil and evil creatures exist not merely because he is permitted to, but because he is prevented, until now, by God from becoming nonexistent). Evil is a deprivation in this view (e. g. in the case of murder, what are missing from the murderer are the following: legality of his action; what ought to be his motivation in living his daily life; and more).
It also means that God Himself is the ultimate goal of human beings (i.e. to behold His uncreated glory and to be continuously transformed by Him, which, according to the traditional view, inevitably leads us to happiness and increases it).
This is why human beings must do good; it is because to do so inevitably leads to that goal: God Himself. If one already beholds God's uncreated glory, it is impossible not to be happy. And to be led to Him is to grow closer and attain Him (though, according to Christianity, this had become impossible due to sin, so Christ came to fix this problem).
Now as for God commanding the slaughter of even the most innocent in the Old Testament, I honestly do not have an adequate answer. For now, my answer will be this: God's commands must necessarily lead to the ultimate goal of mankind as mentioned above.