I love this topic, but I usually have a hard time finding people receptive to it. Generally, the "god is good" catechism is so deeply ingrained in our culture that Satanists are almost universally vilified.
Practically speaking, god is good because god is an omnipotent entity (or claims to be) and has the keys to heaven (or claims to). Satan tried to usurp him, failed, and so was cast into the corner of the universe farthest from heaven and god's presence, what we call hell. Satan is not essentially evil, nor is god good, but their positions make the latter inherently more appealing to us humans. After all, heaven is bless while hell is torment. It's the same logic behind why everyone loves bartenders and there's a stigma for hanging out with funeral parlor staff.
Morally speaking, it's arbitrary, at least so far as I can tell. God is a complete asshole in the Bible: he murders, rapes, and burns, tricks people, forces people to kill each other, incinerates entire nations on a whim, and then murdered every single person on the entire planet. And that's only what he's done in this cycle; who knows what happened to his last creations (since god is infinite, there must logically be a "previous attempt"). On the other hand, Satan uplifted humans from ignorance and servitude, granted us knowledge, and, most importantly, gave us the power to choose our own fate. The fact that he doesn't condemn us to hell for not believing in him is a nice bonus as well.
Just looking at their actions, Satan is obviously the good guy and god the evil guy. But god has the keys to heaven, so we're indoctrinated to love him and revile Satan.
Fascinating stuff, really. Working through this stuff really gives you a deep look into the primal human mind.