How can things considered bad when done by humans become good when God does them?

Requoting your example and adding emphasis and the previous verse:

4 They were told not to harm the grass of the earth or any plant or tree, but only those people who did not have the seal of God on their foreheads. 5 They were not allowed to kill them but only to torture them for five months. And the agony they suffered was like that of the sting of a scorpion when it strikes. 6 During those days people will seek death but will not find it; they will long to die, but death will elude them.

Paul speaks to this in Romans 3:5 (NIV)

5 But if our unrighteousness brings out God’s righteousness more clearly, what shall we say? That God is unjust in bringing his wrath on us? (I am using a human argument.)

The old testament is full of God's wrath against 'sentient human beings' who disobeyed and turned away from his law. How can we expect it to be lesser after he sent is own son to suffer so that we may be with him once more, and we still reject him?

Is justice immoral?

Some might accuse God of being hypocritical or tyrannical when he applies a certain moral law on man that He, Himself, is not obliged to keep, but, I think we all know that to not necessarily be the case when we consider certain human examples.

As with all metaphors, this one's not perfect, and there will be ways to pick it apart, but hopefully the message will get through. The place I see this most clearly demonstrated is in how I relate to my children. There are certain things that they are most definitely not allowed to do (like mess with the lawnmower...or fire up the grill), and as an authoritative head over them, I am not only in a position to break those laws (the very ones I demand of them!) without necessarily being immoral, but in some cases, where I'm actually remiss in my duties if I don't do those things from time to time (like messing with the lawnmower and firing up the grill :) ). I think we all agree that, even limited to the human scope, there is a certain hierarchy in which certain authorities supersede laws imposed on...for a lack of a better term...underlings. Sure, this premise can, and often is, abused, but the abuse of it does not necessarily negate its validity.

According to the Bible, God is supreme authority, and Creator of all things. He gets to set the rules of the game,as it were, so as Caleb suggested, He, is, by definition, the moral standard. We can argue that that's not fair, and that things are stacked in His favor because of this, but our pleadings and desires don't change the nature of this scheme. That's to say, even if God were cruel or capricious, it's His right to be so. Fortunately, the Word He gives us assures us He's not, and He demonstrates His love for His creation through profound displays.

A couple of things that have helped me come to grips with this are presented as illustrations in scripture.

Several places God is referred to as an author of creation and of our life stories. If we consider the role of an author w/r/t his narrative--his created universe--then I think most people will agree that the morality of the author is transcendent (as it were) over the morality of the universe within the pages of his story...by that, I mean, that a certain morality exists within the story, but the author is not bound to that same morality...especially as it impacts the characters he created. He owns his characters and supplies their very existence, so if he kills off characters (even the likable ones) in horrific and seemingly unfair ways, his readers and fans might get upset, but there's no real moral injustice being committed...From that perspective, he's simply woven a tale that some deem unfortunate or unpleasant.

In another place God is referred to as the potter, and us his wares. Once more, this illustration reminds us that God's supreme eminence over his creation means there is no moral culpability to his creatures. If a potter wishes to make a fine vase designed for the very purpose of being elevated and pampered, and then the next day make an ash tray, designed for the very purpose of catching and containing burning refuse throughout its "life," then that's the potter's prerogative, and no one can really fault him for that inequitable design.

Things get a bit tricky -- as they should (lest we become cold sociopaths) -- when these "pots", and "characters" are our fellow man. We should be troubled when we see injustice, and we should weep with out friends and neighbors when tragedy strikes...and we should seek answers from the Great Author to see what it is we're to learn from these harsh events. But we should also understand that these things are His prerogative, and He's not immoral when these things -- even of they are the results of men behaving badly -- happen on His watch (as it were).

This is a somewhat difficult philosophical paradigm, but it's a fundamental one to the Christian.

I will answer this in the context of the scripture reference you provided.

Revelation 9:4-5 And it was commanded them that they should not hurt the grass of the earth, neither any green thing, neither any tree; but only those men which have not the seal of God in their foreheads. And to them it was given that they should not kill them, but that they should be tormented five months: and their torment was as the torment of a scorpion, when he striketh a man.

Because God is just.

The sin in the garden required a blood sacrifice. Either we had to die, or someone would have to die in our place.

EZEKIEL 18:4 Behold, all souls are mine; as the soul of the father, so also the soul of the son is mine: the soul that sinneth, it shall die.

To explain the atonement, imagine that your good friend was guilty of a crime, and you were the judge. You might think it would be just to simply rip up the charges and tell your friend to go home, but in God's economy that doesn't work.

What crime did we commit against God? We were born wrong (through unbelief of God's Word), as a result of Adam and Eve's fall in the garden. Everyone on earth is guilty, even if you have lived a life as pure and clean as a Pharisee.

PSALM 51:5 Behold, I was shapen in iniquity; and in sin did my mother conceive me.

God can't simply rip up the charges and let you go free, because that would not be just. The sin debt must be paid. So He came Himself and paid it by giving His own life.

Now to your question...

What if you turn down such marvelous grace? In the context of the scripture provided, the men with the seal of God had accepted the grace of God, and been sealed away (they were *just*ified). What would be the just thing to do with a sinner who rejects the grace of God?

I must dissent from the proposition that God is full of both love and hate. God is so full of love, that He came and died to redeem us back to Him. But God is also just, so righteous that rejecting His love and grace seals you away from Him.

Is Revelation 9:5 unjust? How about this question: Is it just for a righteous God to send a sinner who has knowingly turned down His grace to hell? From what I understand, hell makes all other torture and torments seem like a toothache.