Possible Scenario I:
Upon first encountering the downed Nightfury, squeamish protagonist Hiccup overcomes his misgivings and stabs Toothless (the dragon) in the neck. A dizzy mixture of sick and satisfied, he brings his father Stoic out to look at the corpse, and instantly gains acceptance into the Viking clan. Downing the Nightfury, which has never before been seen, much less killed, earns Hiccup enormous prestige in the village. Queries about how he did it soon turn to praise for his dragon-capturing device, and Hiccup's considerable mechanical talents are directed towards forging new weapons of war.
The village is able to drive the lesser dragons away and attain a stalemate with them; the other dragons compensate for the decline in sheep (which they feed to a giant dragon overlord) by catching more dolphins, sharks, swordfish, etc. to feed to the overlord dragon instead. This results in a boom in the oceangoing predators' prey fish populations, making the village prosperous for the first time in years. Meanwhile, Hiccup remains deeply troubled over the death of the Nightfury, but his quiet suggestion that the dragons might be more than just monsters goes unheeded. Forced to keep on killing the creatures he secretly loves, Hiccup eventually commits "suicide by dragon" and allows himself to be burned to ash during a dragon attack.
Possible Scenario II:
Stoic's ship never returns from his attempt to find the dragon's nest. A grieving Hiccup saddles Toothless and goes out to search for his father, but is unsuccessful. Astrid sees Hiccup leaving on Toothless's back, and spreads the word in the village that Hiccup is riding a dragon. When he returns empty-handed, the Vikings conclude that he's in league with the dragons, and that he told them how to find and sink the Viking ships. Hiccup is forced to flee the village with Toothless and carve out a new life for himself on a neighboring island.
|Hiccup and Toothless are being forced to leave their loved ones behind, forever. That is why they are scowling.|
The movie ends as it does on screen, with Vikings and dragons living in harmony, and the credits roll. Five or ten years down the line, the Vikings (who have been fighting the dragons for generations, and now have no enemies) get restless. Their warrior-based culture begins to come apart. A young, charismatic leader plays on these feelings, and pointing out that the Vikings are now experienced dragon riders, calls for a campaign of conquest against all nearby settlements, using the dragons as weapons of war. Hiccup frantically tries to stop the crusade, but is powerless to do so, and can only watch as the pets he knows and loves slaughter thousands of people in other, nearby settlements (and eventually all the way to Britain).
Ain't it fun??
(Bonus scenario, if this movie was made in a culture with different values: Hiccup learns to accept that while he may not like his father's path, it is the one he is obligated to follow, and eventually becomes a productive, reasonably normal member of Viking society. Instead of setting out to change his home culture, he bows to it.)