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Answer by Leonard Wei:
I actually want to dispute the premise of this question.
I think the reader tends to actually dislike Jaime Lannister a bit more than the average hypothetical person in Westeros. We know a lot more about him, and our first impression definitely didn't go over well. To this day, I'm still surprised that I think of Jaime as not that bad a guy. That feeling should be understandable to anyone who has read through at least the fourth book. That being said, he pushed Bran out a window!
Back to the Westeros peasant's standpoint. He's a knight of great renown; he's one of the most famous tourney knights of the time; he's a member of the kingsguard, an elite order; he's a Lannister; he killed the Mad King, who was a bad person according to just about everyone. Bran actually thinks of being someone like him when he grows up. His label as the Kingslayer definitely makes certain people of honor sneer at him, but overall, I think he is loved by the commoners.
As the series goes on, we just see him among people that despise him exclusively. He's captured by the north, so everyone around him hates him. His family is responsible for the death of Ned. He personally killed many people during his capture (including two of Karstark's sons). The rumors of incest with his sister also turn his reputation more sour, but the entire lens we see him with is tinted by the northern viewpoint. In a more neutral light, I think most common Westorosi look up to him and admire him as a great knight.
Answer by Matt Johnson:
My perspective on this relates to how Jaime can actually feel guilt for the Kingslaying act and yet feel nothing when trying to kill Bran:
Jaime's feelings simply don't make sense to us. Yes we can state them, but we don't really get it. The closest any explanation comes to my mind is by saying that Jaime actually has no honor, that he simply dislikes how people treat him. I believed this at first, but Martin's put enough effort into the character that he's clearly not meant to be someone who is universally bad, so we have to accept this apparent discrepancy.
Now apply that to everyone else: If Jaime actually feels guilt over this action we see as righteous, and other people seem to hold it against him, they probably have a similar sense for why that action was somehow bad.
An interesting parallel to this is Huck Finn's perspective on slave Jim. Huck's "moral" beliefs are those of his time and place, and yet Huck can't bring himself to, say, turn Jim in because of his emotional connection to Jim. Huck thinks he's doing something wrong ... meanwhile the reader wants to scream to him "No, you're doing the right thing and the fact you have this connection to Jim says exactly why you're wrong and your society is wrong," but none of this changes the fact that Huck would do the "right" thing if he could make himself do it.
We see what Jaime did (in killing the Mad King) as a good thing, and deep down, he and others don't. That's incredible.
With that said, I don't think things are quite so clear-cut. Hard to imagine that common people really see this as so wrong even in Westeros. As is correctly hammered in all throughout the books: Much of the "honor" of the aristocracy is seen as BS by the common folk. For these people then, "Kingslayer" is more of a nickname than an insult, and why not, it's by far the most notable thing he's ever done.
Additionally, I think the connotation of Jaime as Kingslayer has much to do with Jaime himself. Jaime's not just any Kingsguard, he's a Lannister, and he carries himself with all the arrogance that that name implies.
Had someone like Ser Barristan killed Aerys, chances are he isn't anywhere near as reviled as Jaime is.
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