This is the state of those who come before God with hard hearts. Because of their hard hearts, they will have done and said thinks contrary to the gospel, and those things will condemn them. It is interesting that Alma shows here the nature of the condemnation of God. It is more of a self-judgment. It is the person before God who dare not look upon him. It is not God who exercises condemnation, but the person himself.
Literary: It is possible that Alma is making a glancing reference to a passage in Isaiah when he discusses the desire to hide of the person with sin before God:
Isa. 2:10 Enter into the rock, and hide thee in the dust, for fear of the LORD, and for the glory of his majesty.
11 The lofty looks of man shall be humbled, and the haughtiness of men shall be bowed down, and the LORD alone shall be exalted in that day.
The imagery is of one hiding in the hiding places outside Jerusalem, which were traditionally multiple caves. In this case, Alma has amplified the imagery, and instead of simply hiding, the sinful person will want to be so far hidden from God that he will want the mountains themselves to be on top of him, that is, between himself and God.