Rhetorical: Alma is simultaneously addressing Zeezrom and the crowd. He is answering Zeezrom's unstated question, but he is also attempting to reinforce his comments to the crowd. To understand how he is doing this, we need to return to an earlier verse:
Alma 12:6 And behold I say unto you all that this was a snare of the adversary, which he has laid to catch this people, that he might bring you into subjection unto him, that he might encircle you about with his chains, that he might chain you down to everlasting destruction, according to the power of his captivity.
The words "chain" and "captive/captivity" and been italicized in verses 11 and 6. Alma is picking up on the themes he used in verse 6 when addressing the crowd and using them again in a slightly different context. In verse 6 the lawyers had the people in chains and captivity. Now Alma uses those terms for the chains of hell. There is a strong implication drawn that the lawyers are cohorts with the devil. While they might proclaim their belief in God, Alma associates them with a very different being.
We cannot be certain why Alma brings up this reference to the chains of hell. It is not required by the context of the interchange so far. However, it is a theme that Alma has used in the past, most notably in his discourse in Zarahemla (see Alma 5:7-10).
Doctrinal: Alma gives a definition for the chains of hell that is metaphorical rather than literal. It is instructive to examine the process that he considers the chains of hell. We will analyze verse 11 phrase by phrase:
[And they that will harden their hearts] There is a particular type of person who will be subjected to the chains of hell. The beginning of the process is that the person will harden his heart. This process is one where the person begins to actively reject the promptings of the spirit. The heart is used as the metaphor for the way we feel these promptings, and the hardening of the heart as the process by which we begin to reject the promptings of the spirit.
[to them is given the lesser portion of the word] As a person's heart becomes increasingly hardened, they receive less and less of the word of God. Of course this is not because the Lord no longer offers it, but because of their increasing unwillingness to what they hear as the word of God.
[until they know nothing concerning his mysteries] One of the important terms in this phrase is the word "until." This indicates this is a process. It is not something that occurs overnight, but rather over time. The more one hardens their heart, the less one hears of the word. The less one knows of the word of God, the less one knows of the mysteries. In this case, the "mysteries" should not be considered to be the great imponderables about God, but rather any understanding of him at all. Because God is so much greater than man, all true information about his is a "mystery" in that it is beyond the scope of normal human logic and understanding.
[and then they are taken captive by the devil] Satan steps in where there is a void in our understanding of God. If we will not worship God, Satan will find a way for us to worship him.
[and led by his will down to destruction.] No one who is so gently led down by the devil willingly walks to his own destruction. They go because they do not recognize the path they are on. They go willingly, thinking they are on a very different road. The spiritual destruction will be the same whether they see it coming or not.