We never know what type of sin Dante has committed — this is not important — somewhere, he has simply strayed from the straight path. As he travels through this dreadful region, he retains those qualities that he has always possessed. And, he also displays a variety of emotions ranging from pleasure, to pity, to sympathy, to horror and revulsion.
For example, at the time of his journey, Dante was a rather well-known writer, and when he confronts the great classical writers whose greatness has survived the measure of time, he expresses awe merely to be in the presence of such greatness. Then, when the greatest poets of all time invite him to join them, this is a compliment of such high caliber that Dante's pride is heightened immeasurably.
In contrast, Dante soon meets a Glutton in Hell. Dante remembers him with pleasure. After all, Ciacco was a jovial and gracious host in life and was the typical "life of the party." Dante can only listen sympathetically to his condition. He feels so apologetic for not recognizing Ciacco that he fabricates an excuse so as not to hurt his feelings.
What endears Dante to the reader is his compassion for the sinners, even though he later comes to recognize that his pity is wasted upon them. Likewise, when he sees an enemy in Hell, such as Farinata, Dante is noble enough to recognize the power of the man, even while totally disagreeing with his political views. He sees Farinata as a strong majestic figure "towering" over Hell itself. He responds favorably to Farinata's love of Florence, especially when the sinner acknowledges that "maybe" he tested the city too much, but at least he was the one who kept his colleagues from razing the city to the ground. For this one act, Dante is proud to have met this powerful man and acknowledge his outstanding feats.
As he descends, he finds a beloved advisor, scholar, and fellow writer suffering, and his compassion is unsurpassed. He promises Brunetto Latini that his writings will be kept alive for all people to read and know. He departs from this wonderful teacher with tears in his eyes — it is one of the last times that Dante will weep for a sinner.
Dante, however, is not a one-sided person. He also has the power to respond to certain vicious sinners in a manner befitting their sins. When the wrathful person strikes out wildly, Dante has no pity and would possibly strike back. Then, in the ironic description of the sullen, Dante, for the first time, uses ridicule, and in the next circle he is seemingly pleased when the sufferings of Filippo Argenti are increased.
When a shade in the bottom of Hell refuses Dante's request for his name, Dante reaches out and uncharacteristically hurts the sinner by pulling out a tuft of his hair. Earlier, when he had inadvertently hurt the shade of a suicide, Pier delle Vigne, he feels deep remorse for injuring the sinner.