Symbolism, Imagery, Allegory
Belton Blew it First
Before any of this stuff with Clarissa went down, Lovelace and Belford were gossiping about their buddy Belton's downfall. See, the guy whole-heartedly embraced the rake lifestyle and lived it up for a long time. Then he got sick and his mistress turned on him, leaving him broke and miserable. Now, we're not saying it's related to his philandering ways, but come on—it definitely is.
So how do we know that Belton's story is an allegory for Lovelace's lifestyle? Well, Belford keeps not-so-subtly hinting at the parallels between the two buddies. Lovelace doesn't really seem to get it, though. Not until Belford spells it out for him in all caps: "THOU MUST DIE, AS WELL AS BELTON" (419.7). Message received loud and clear!
So why would Richardson stick in an allegory that basically rehashes the plot? Well, the guy was a moralist. Since the major critique of his other best-seller, Pamela, was that the moral wasn't clear enough, Richardson gave the audience what they wanted.