Iago's characterization as being "motive-hunting of a motiveless malignity" helps to explain the fundamental sense of bitterness and resentment that eats away at Iago. Coleridge uses the term to suggest that Iago appropriates the world around him through a "motiveless malignity:"
The last Speech, the motive-hunting of motiveless Malignity--how awful! In itself fiendish--while yet he was allowed to bear the divine image, too fiendish for his own steady View.--A being next to Devil--only not quite Devil--& this Shakespeare has attempted-- executed--without disgust, without Scandal!
Coleridge's use of the term "motiveless" reflects a fundamental sense of unhappiness plaguing Iago. It is rooted in what Coleridge sees as part of Iago's characterization, and what "motiveless malignity" looks like: ".. the mere fictions of his [Iago's] own restless nature, distempered by a keen sense of his intellectual superiority, and haunted by the love of exerting power, on those especially who are his superiors in practical and moral excellence." Coleridge uses the term "motiveless malignity" to describe Iago's sense of resentment and bitterness at being passed over for promotion, for being denied power, and for not enjoying what he considers to be his rightful sense of happiness or entitlement.
It is this quality that makes Iago so villainous. There is no direct end that will satisfy him. Shakespeare's construction of Iago is one where lives a bottomless reservoir of anger, resentment, and spite that motivates his evil plans. Throughout the drama, lines that Iago speaks can be seen as ironic because they intimate at a condition disarming as to the depth of its bitterness. Consider lines such as “I am not what I am," “Men should be what they seem; / Or those that be not, would they might seem none," or “Let’s be conjunctive in our revenge.” These are lines that tap into the resentment that lingers inside Iago. One gets the impression that Iago will destroy anyone that comes into contact with him. His final line of how he will speak no more is a reflection that only death can curb the level of intense anger and resentment that is within him. The "motiveless malignity" is evident in this vortex of the suffering that he inflicts on others, emanating from a source within him that can never be appeased or soothed. There is always some slight that he suffers from the world, a reality that prevents Iago from embracing a redemptive position. It is to this end that his "motiveless malignity" becomes an essential part of his characterization. Shakespeare's depiction of the evil within human hearts is a disturbing one. It reveals that some people cannot be appeased, and that some are beyond discourse. Some just wish to watch "the forest burn." This becomes an appropriate description of Iago's "motiveless malignity."