To modern readers, Aeneas can seem like a bit of a tool. We mean this in both a non-technical and a technical sense. In a non-technical sense, Aeneas is a tool because can come off as a bit of a jerk. At least, this is what it might seem like when he first shacks up with Dido, giving her the impression that they're married, and then suddenly sails off on his mission, telling her, "Sorry, gotta go. And hey, it's not like we're married or anything." When he meets her in the underworld, he tries to strike up a conversation like it's all no big deal – and then tells her again that it wasn't his fault. Come on, Aeneas, even if it wasn't your fault, you could at least show a little bit more empathy! Don't you know that the bigger man admits when he's wrong?
Aeneas's not-so-nice side reveals itself again in Book 10 and Book 12, when he kills various guys (including, finally, Turnus), who are surrendering and begging him for mercy. OK, so you were really angry that Turnus killed Pallas, but still, don't you remember your father telling you to "spare the conquered"? Is that really the example you want to be setting for your son Ascanius and the countless generations of Romans who will come after him?
That was the non-technical sense in which Aeneas is a tool. Interestingly enough, the technical sense makes the non-technical sense seem not as bad. (At least, this seems to be the point of the poem, so we tender-hearted moderns just have to find a way to wrap our heads around it.) Aeneas is a tool in a technical sense in that he is an instrument of fate. He didn't up and decide to go off on a mission to found a new city: it was the gods who put him up to it. Deep down, Aeneas has feelings too – including feelings for Dido – but he knows that he has to smother those feelings in the service of a higher cause, just as he smothers his feelings of grief for his lost companions when he puts on a brave face for the survivors in Book 1. This is what...