The dictionary explains that the word "beset" means "to surround, hem in, or continually harass." This is precisely what sins do. They surround our thoughts and feelings of self-worth. They hem us in and keep us from our personal potential and the presence of God. They continually harass us until the spiritual cancer has festered to our destruction. From the above, one might think that the power of sin to beset us is greater than our power to resist.
Yet Alma uses an interestingly casual phrase to describe our response to sins—we are to simply lay them aside. Can we lay aside sin as easily and casually as we discard garbage, throw out the weeds, or remove roadblocks? Apparently, we have more than enough power to resist all the fiery darts of the adversary (DC 3:8). This concept is encapsulated in the word of the Lord to Satan, "you shall have power to bruise his heel, but he shall have power to crush thy head." The individual who is born again has no more desire to do evil (Mosiah 5:2). Therefore, they have the power through their own agency to discard those besetting sins. Alma's imagery is appropriate because it is these sins which act as roadblocks on the strait and narrow path. We must lay aside everyone of them so that we can move forward.