In Norse paganism, Baldur is usually seen as he was described in the Eddas, as an innocent god who was killed by mistletoe due to Loki's schemes. This particular myth has several versions, most variations regarding Loki's motives and deeds (most recall this story as a proof of Loki's evil, but especially in earlier versions he usually only did so due to the way he was treated by the other gods, and while some versions claim the giantess who refused to cry for Baldur was a disguised Loki earlier versions don't claim such) and how the gods reacted to Baldur's death (some versions only show them punishing Loki because he insulted them, not because Baldur died, while in some versions a particularly sadistic Thor throws a dwarf into the funerary pyre of the dead god).
However, lesser known is that Baldur was involved in other recorded myths, all portraying his death. In one, recorded by Saxo Grammaticus, Baldur was the rival of the mortal Hother/Hoddr, who in the Eddas is the blind god who killed Baldur accidentally via Loki's guidance. Here, they both wish to marry the princess Nanna, daughter of Gewar, king of Norway. Being the son of Odin, Baldur clearly has the advantage in his side, but Hother manages to win and marry the princess. Baldur clearly is not satisfied, and later tries to take away Nanna. To kill off the god for good, Hother travelled to the Underworld, paralleling the journey Odin or one of his sons had to do in the more famous myth to convince Hel to release Baldur. There Hother was given a sword named Mistilteinn ("Mistletoe"), which he uses to kill Baldur. After three days of agony due to his wounds Baldur dies.
A similar tale was recorded in the Chronicon Lethrense and Annales Ludenses, where Baldur is killed by Hother (here the king of the Saxons) while chasing away Odin and Thor. As revenge Odin sends his son Both to kill Hother. Because the story is very confusing and everyone kills each other for no good reason I will focus my energies instead on Saxo Grammaticus' story, recorded in Gesta Danorum.
While different, Gesta Danorum's tale and that recorded in Poetic Edda and Prose Edda are similar in some ways. In both, Baldur, the divine son of Odin, is very popular, loved by most, while his rival isn't as popular. In Gesta Danorum, Baldur has the gods of Asgard on his side against Hother, and in the Eddas Baldur is loved by everyone, to the point every single thing except mistletoe (and only because Baldur's mother underestimated it and didn't asked for it to not harm her son) promises to not hurt the god, while everything except an old giantess (depending on the version, she might had been Loki in disguise) cries for him to come back. In both versions, said love everyone has for Baldur allows him to be invulnerable to nearly everything; in Gesta Danorum regular weapons harm him not, partly due to Odin's will and partly due to Baldur's divinity, and again practically everything promised to not harm the deity in the Eddas.
In all stories, Hother is the killer of Baldur. In the version recorded by the Eddas, he does so accidentally; being blind, his hands are guided by Loki and thus he pulls the bow and throws mistletoe at the god. In Gesta Danorum, he intentionally kills Baldur. Whereas he is sympathetic or not depends on your point of view; in both versions he is essentially underestimated by everyone and not very liked, a contrast to Baldur's popularity and excessive support. Note that in Gesta Danorum Hother only kills Baldur in their second battle, after the god arrogantly declaring the first battle wasn't fair because he hadn't won.
And finally the parallels between both stories occur in the journey to the Underworld; in Gesta Danorum Hother goes to obtain Mistilteinn, while in the Eddas Odin or one of his sons goes to retrieve Baldur. Considering Baldur is the god of light, a journey to the land of the dead (usually considered a dark and cold place) is symbolic for different reasons; in the version of the Eddas, it could probably symbolise the death of the Summer god during the winter, a motiff found in several pagan religions. In Gesta Danorum, it could be seen as that darkness ultimately triumphed over light, and its not really a bad thing because the god of light was a jerk just like most gods. The weapon that kills the god is basically known as mistletoe, but while in the Eddas it was literally mistletoe, in Gesta Danorum it was a sword with that name.
Because mistletoe naturally doesn't grow in Iceland, and because actual mistletoe killing a deity sounds silly, it is possible that Gesta Danorum contains the original myth, if only because the nature of Baldur is more akin to that of a real god, as opposed to the Eddas where he is basically Jesus with another name.
Baldur stands for the childhood of the western world
When Roman civilization was pretty much destroyed during the völkerwanderung, people pretty much had to start from scratch. About 99% of the people, even most rulers, were illiterates. Their education was on a lower level than that of a child today.
But around the time the Edda was written down, civilization started to take off in western Europe again, with the 12th century renaissance (yes, this exists, and it came before the more famous renaissance). While this was great in many ways, it also meant: The days of figurative childhood were over. Baldur was the god of youth, and he is killed in the story. You see the connection?
Now while no author from this time could have consciously written such a story for the lack of universities, widespread media and TV Tropes, he still could have done unconsciously.
The Northern Spirit
Basically the ancient Teutons were the poster boys (and girls) for the philosophy 'Life isn't Fair' and their myths reflect it. The relative precariousness of life in the northern latitudes as opposed to the Mediterranean seaboard probably had a lot to do with Teutonic fatalism. Their prime virtues were courage and fortitude in the face of inevitable disaster. Even their Gods were ultimately doomed - but if you were brave you got to party in Valhalla before dying in the Final Battle.