If Calvinism is true, then God causes all things to exist, even evil. Since there is no such thing as libertarian free will, there are no independent causal powers apart from God’s own. Therefore, all that comes about comes about because God specifically intended it to do so (unless you want to claim that God causes things he does not intend.)
What this means is that God must be pleased to bring evil about. Since evil could only exist by God’s decretive will, God must will that it exist. And, since God does all that pleases him, it must please him that evil exists.
How does this square with the idea that God hates evil? On Calvinism, God creates vessels of wrath because he is pleased to destroy them — but he destroys them because he hates evil. What you have here is a situation in which God is both pleased with the existence of evil and displeased with it. But this seems a contradiction: insfoar as evil exists God must want it to (for nothing exists without him willing it to exist); yet insofar as he destroys evil he must not want it to exist.
I have seen at least one Calvinist (Sam Storm) attempt to answer this problem by saying that “God is pleased to ordain his own displeasure.” But apart from any sort of notion of divine self-limitation, I fail to see how this quote is not a bare contradiction. For what you are saying is that God is pleased – i.e. he delights in – doing what displeases him – i.e. what he does not delight in.
Once again we come to the fact that, if we are going to maintain the reality of evil, and if we are not going to simply say that all things happen by God’s desire and will (which would make all things actually good, since God’s desire and will are themselves perfectly good), then we must posit some notion of creaturely free will. Otherwise, if God is the immediate and single source of all, then he is the source of evil as well. But if he is the source of evil then either a) there is no such thing as evil (for God does only what is good); or b) God is not all good (since God is pleased to intentionally create evil.)
Now, if we say that God is not pleased to create evil, and that it is a source of hatred and wrath for him, then we must ask why it exists at all, seeing as it is ultimately God’s will alone that is the cause of things. Does God will that which he hates? And how could he hate that which comes specifically and intentionally from him alone, the all good perfect being?
Again, if God’s will is never frustrated on Calvin’s scheme, then what is God’s relation to evil? If it is one of hatred, then his will is in some sense frustration, since one does not hate what one desires. If, on the other hand, it is one of delight, then God’s will delights in evil, which either makes his will less than all good, or makes evil cease being evil altogether.
Either way, one is driven to a contradictory conclusion.