It's time to defend something like Anselm's idea of infinite culpability. I once expressed a hope that I'd eventually do so, but couldn't think of good things to say. Anselm's idea was that there was an infinity in our sins, because they were sins against an infinite God. Here, I just want to defend the real possibility that a human being is infinitely culpable.
Case 1. Mason justifiedly believes (correctly or not) that the universe contains infinitely many inhabited planets, each of them with rational beings as deserving of respect as humans are. Mason works as a janitor at the Large Hadron Collider, and also justifiedly (but incorrectly) believes that if he loosens some bolts, the Collider will malfunction in such a way that it will destroy the universe with all its rational beings. Mason wants destructive power--he wants to outdo Hitler and Stalin in their destructiveness--and so he loosens these bolts so as to kill infinitely many rational beings. In so doing, he commits attempted murder of an infinite number of people.
Now, in most nearby possible worlds where Mason does this, he is likely so insane as not to be fully culpable for his action. But it is within the bounds of real possibility (causal possibility consistent with the basic structure of the world and humanity as we know it) that Mason is not so insane as to fail to be culpable.
Case 2. Lara justifiedly believes (correctly or not--I think correctly) that there is a heaven and a hell, and that hell involves eternal suffering and heaven involves infinite eternal bliss. She hates Samantha and tempts her into a serious sin, in order that Samantha would suffer forever in hell and lose the infinite joy of heaven.
For evaluation of Lara's culpability, it doesn't much matter whether her belief about hell is correct or whether she succeeds in destroying Samantha's soul. Lara has acted so as to make Samantha lose an infinite good, and that is an infinite culpability.
Case 3. Alex justifiedly believes (correctly or not--I think correctly) that memories of moral goods had in this life contribute to the joy of heaven on infinite numbers of occasions, adding an infinite amount of joy. He acts in a way that makes someone lose a moral good in this life. According to his beliefs, he has acted in such a way as to have made someone lose an infinite number of goods. Assuming he was sufficiently aware of this when acting, he is apt to have an infinite culpability.
What is helpful about Cases 2 and 3 is that the beliefs in them have some chance of being correct.
Question: Are there flip-sides of these cases that show that we can gain infinite merit?
Answer: This is not as obvious as it may seem. For while it is twice as morally wicked to commit an act that kills two innocents, it is not twice as morally good to commit an act that saves two innocents. If by giving a dollar I can save one life, and I do so, I have shown a small amount of virtue. But if by giving a dollar I can save two lives, and I do so, I have not shown any more virtue, since I did something that was more strongly required and at no greater cost. On the other hand, had I refrained from giving the dollar, in the two-life case I would have done something about twice as bad. So one does not generate cases of infinite merit simply by supposing beliefs about infinite goods.
Nonetheless, one can manufacture cases of infinite merit by supposing beliefs about infinite sacrifice:
Case 4. Chuck justifiedly believes that (a) if he helps a slave escape, he will suffer infinite pain in hell; but (b) it is his moral duty to help the slave escape. In that case, there is a kind of infinity to the merit of Chuck's helping the slave escape. (There will, of course, be questions about ulterior motives. If he does it to have a self-righteous feeling, there may not be such merit. So this suggestion does not do away with the idea that one needs grace for infinite merit.)
(I don't know how close the case of Chuck is to the much-discussed case of Huck.)