Armed with the knowledge that mistletoe can kill Baldur, Loki sets out to have some fun. His brand of fun, anyway. He collects the mistletoe, forms it into a dart, and approaches the God Hodur (not to be confused with Hodor of Game of Thrones fame).
Here we begin to see Loki’s sadistic sense of humor truly emerge. He pretends to show concern for Hodur, wanting to know why he’s not participating. Hodur is blind, so can’t see where Baldur is, and likewise has no weapon. Loki to the rescue! He makes the mistletoe into a . . . well, translations can’t agree. Some say dart, others say arrow, and also spear. I’ve seen pictures of mistletoe, and I find it hard to believe that someone can form a spear out of it. Whatever it is, it’s a thrown weapon. Loki generously offers to help Hodur line up the shot, and, blammo, Baldur is struck. Dead. Instantly.
Now, why did Loki do this? Simple, it’s funny, for one thing, on multiple levels. One, this is the plant that was said to be harmless, there’s irony for you. Two, the gods were challenging anyone to find a weapon capable of hurting Baldur, turning it into a game. Hodur is outcast from the rest because of his blindness, but Loki brings him back in under the guise of inclusion. He is masquerading as a friend to give Hodur the same honor that all the other gods have in this game. Essentially, Loki is mocking all the actions of the gods and their honorable game.
Even the pretense to help Hodur is nothing more than to help himself. After Baldur is killed, Hodur falls out of the mythology. No one wants to even speak his name because, despite Loki suckering him and providing the weapon, despite Frigga’s arrogance that mistletoe could harm no one and spilling the beans to Loki, and despite the arrogance of all the other gods to gladly throw weapons at Baldur, Hodur holds the blame for this action. His hand held the weapon. His throw killed Baldur.
This isn’t to say that Loki is off the hook. They’re after him, but he slips away temporarily—he’s good like that. This also shows the lengths Loki will go to play a practical joke. This one is a doozy, too, making fun of all the gods in the process, and brining Ragnarok one giant leap closer. But, hey, it’s all in good fun, right?
We also have to look at Odin in this. He’s not mentioned specifically as being present for the event, but he finds Loki afterward since he has gained knowledge of the future. Yet, this knowledge is not perfect. Odin deceived himself, in a way. He believed Baldur to be safe for time even knowing that Baldur had to die before Ragnarok. Because of fate, Ragnarok cannot be avoided, merely delayed. So while Odin knew Baldur would one day die, he thought that day far off because of the oaths. Loki’s deception proves to be the loophole that Odin’s visions of the future cannot account for. Again, Loki has some awesome power by being able to lie.
BONUS! I’m going to drop some science. The choice of mistletoe as the plant that kills Baldur actually makes a good deal of sense as it is a parasitic (vampire!) plant. It steels the life from other trees in order for it to live. While it rarely completely kills off a tree, it can do so. The Norse would have readily known this, so it’s fitting that a plant that takes the live of a tree would kill off Baldur.
Everyone should seriously reconsider kissing under this plant at Christmas. Just saying.
We must also remember the important symbolism of trees in Norse mythology. The World Tree Yggdrasil connects the nine worlds, and humanity is made from the ash and elm tree (man and woman, respectively) showing the importance of life from the trees. So, truly, in terms of deadly yet unassuming weapons, mistletoe is one of the best out there.