Dante cares deeply about the moral thought processes of mankind, having personally suffered as a result of others’ sin. He was exiled from his hometown. We know the intensity of his feelings is often obscured by his fancy style, but you can’t have crying and fainting and damning from the first-person narrator without running a pretty high emotional fever.
The "sympathetic" part comes out in the portrayal of the so-called "noble sinners" – souls like Francesca, Farinata, Brunetto Latini, and Ulysses – who speak very little about their actual sins. Their stories are designed to make readers ask why they are in Hell and, often, Dante’s reaction is the same, plus some weeping and swooning.
However, Dante is particularly pitiless with the fraudulent sinners. When you have lines like, "O Simon Magus!...Rapacious ones, who take the things of God, / that ought to be the brides of Righteousness, / and make them fornicate for gold and silver! / The time has come to let the trumpet sound / for you…" you’ve got some serious damning going on. Notice that these lines actually have no quotes around them in the original text, meaning that it is not character-Dante who speaks them, but our omniscient tone-setting author-Dante.