Markheim's nature reflects human nature in general as he has both good and bad in him. Humans, in general, commit both good and bad acts. We might be tempted to do foolish or bad things (although not everyone, of course, is capable of such a serious crime as murder), but we also have the capacity for regret and remorse; we all experience internal conflict as Markheim does.
Markheim commits a wanton act of murder but by the end he feels enough remorse and regret to voluntarily hand himself in. Thus, he achieves some measure of redemption. Furthermore, he gives himself up despite the temptings of his strange visitor who urges him to kill the maid who comes to the door and ransack the entire shop for money.
There has been much debate over who the visitor is meant to be. Markheim thinks at first that it is the devil, but his radiant transformation upon Markheim's decision to pay for his crime suggests far otherwise:
The features of the visitor began to undergo a wonderful and lovely change: they brightened and softened with a tender triumph; and, even as they brightened, faded and dislimned.
The visitor, then, takes on a whole new, 'wonderful' aspect before fading, or 'dislimning'. This implies that this being is a spirit of some sort; maybe, as many have argued, it is Markheim's conscience made visible. In any case, the central point is that Markheim comes to repent, and of his own accord.