Actually… I think that Norse mythology is even more interesting. Obvious caveat for this being me recalling most of it and checking only here and there if I'm truly correct, but at the worst case I'm not far from the mark.
Turns out that in Valhalla, even the gods can die. As in truly die. They don't return. And this is the story of why Loki is such a bastard. The story of Höðr and Baldr, two sons of Odin and Frigg.
Frigg had a vision one day. One where Baldr will die forever. It was ludicrous, as Baldr (god peace, love, forgiveness and justice) was loved and admired by everyone. He was invulnerable to almost everything to boot. Just to be sure, she walked to every man, god, creature or spirit in the realm and asked them if they would ever strike her son. To be even more certain, she put a strong magical protection over almost everything in the land that it will not hurt her son. Surely no-one would willingly strike him down! Everyone she asked denied to ever have such idea. Admired and loved he enjoyed the life among gods and men, feasting, battling and being badass Vikings.
But his brother, Höðr, could not join others in their doings and was left out. He was the blind god, and for this reason he could not participate in most aspects of his expected life. It's important to note that he did not resent others, but felt only sadness and melancholy for not being able to spar, hunt or similar activities.
Until one day! Höðr was sitting somewhere alone, others were sparring or doing whatever gods do. Loki came to him and asked why Höðr is always alone, and to that he replied that due to his blindness it will never be possible for him to join in on any games. Loki offered him to be his eyes and handed him a bow. Some said he guided the arrow itself, others say that Loki simply guided Höðr's bow in a way that it would hit Baldr. It did not matter, for like that he was slain. What matters is the fact that Baldr was invulnerable to nearly anything. And Loki gave Höðr the bow with mistletoe arrow.
What's so important about that? Easy. Remember how Frigg was asking everyone who came to her mind. And she forgot to ask mistletoe. She did not remember about it during the magical protection being put into the effect.
After hearing what has happened to Baldr, Odin and Rindr (a giantess) made a new god Vál;, his sole purpose of killing Höðr. Váli grew to adulthood within a day and murdered his half-brother. It was justice served for killing its embodiment.
Frigg however, was devastated and she wanted Baldr back. She went to the underworld to ask Hel (Loki's daughter no less!) for her son. Hel replied "If you will ask every being alive or dead that exists in the world and they will weep for him, I will return Baldr to you". Loki too guise of another giant, Þökk, and told Frigg that he will not weep for Baldr. Thus, he had to return in Hel's realm until the Ragnarok.
Anyway, gods got pretty pissed off at giants for that one. Baldr would be back, if not for one of them.
This is how the Ragnarok began.