Kurse's sword in Thor: The Dark World is not poisoned. The movie was likely attempting to depict Kurse's combat-laden history and his weapon-mastery as well as his superhuman status after his transformation. The only thing we can be sure of about the sword is that it is not made of iron, which is deadly to normal Dark Elves.
From the Frigga entry on the Marvel Movie Wikia
When Asgard is attacked, Frigga fought to defend those that she loved at any cost. She fought Malekith while defending Jane Foster. The battle was interrupted by Kurse who restrained her. As Malekith threatened Jane she was revealed to be an illusion. After questioned, Frigga refused to tell her enemy anything, and Kurse stabbed her through the heart killing her instantly just as Thor came in and angrily shot Malekith in the face with lightning, scarring him. However, Malekith and Kurse escaped. Thor turned to see Odin holding Frigga in his arms. She was given a funeral and burned as the Asgardians mourned the death of their queen.
This is likely to be a medium-translation error. Without a history to show it, the character (Kurse) has to be able to show how much of a threat they he is to build up to his physical confrontation with Thor. He is shown in the subsequent fight to be easily capable of holding his own in the battle with the Thunder God.
The comic appearance of the Dark Elves had an attack on Asgard but primarily on Thor. In the pastiche created by using the Malekith and Dark Elves and mixing them into the MCU, in the movie's depiction of Malekith's attack on Asgard, the character Kurse also appears but acquires his powers differently from the canon Marvel Universe.
Algrim the Strong (left) the transformed Algrim into the one of the "Kursed" a magical soldier-warrior with increased strength and superhuman capabilities.
Marvel Canon Earth-616
As all the Dark Elves, Algrim (who would become Kurse) was born in Svartfaltheim, home of his race, and he served in the army of the ruler and sorcerer Malekith the Accursed. Fighting under his lord, Algrim distinguished himself among all his peers, and earned the name “Algrim the Strong”, one of the deadliest Elves at Malekith’s command.
His prowess in battle made his master notice him, and Malekith himself summoned Algrim, and telling him he had been chosen as the one who would have killed Thor Odinson, prince of the Asgardians and enemy to Svartfaltheim.
Even though the Asgardians are quite hardy, killing Thor requires someone whose superhuman strength would be nearly equal to Thor's. The Prince of Asgard is three times as strong as any average Asgardian and as a result anyone capable of fighting hand to hand with him would be much more likely to cause catastrophic injuries even to the hardy citizens of Asgard. In the comics, Kurse had twice the strength of an unenhanced Thor, capable of pressing 200 tons.
Kurse possesses a number of superhuman attributes as a result of his natural dark elf physiology and mystical augmentation.
Kurse's main advantage against foes is his strength - while initially a close match for Thor, his strength was first increased to twice, and then four times, that of Thor.
Marvel Cinematic Universe version of Kurse
Kurse in Thor: The Dark World did not function in quite the same way as his comic namesake but was enhanced in a fashion to make him incredibly formidable, easily the physical equal of Thor.
Algrim was a lieutenant of the Dark Elf leader Malekith. Loyaly serving his master during the war against the Asgardians, he was one of the few Dark Elves who survived the catastrophe that almost wiped out their race.
Five thousand years later, Algrim didn't hesitate to sacrifice himself to transform into the last of the Kursed, a Dark Elf "super soldier", so they could unleash their revenge on Asgard.
Kurse's powers were very similar to the canon Marvel-Earth's version of the character. He was easily capable of fighting Thor in hand to hand combat. Most normal Asgardians would have no chance of winning a hand to hand struggle with him.
Algrim, as a Kursed, was one of the physically most powerful beings within the Nine Realms that could easily supprass Thor's strength (a feat acquired by very few).
Superhuman Strength: The level of Algrim's original strength is unknown, but as a Kursed, he is the strongest of the Dark Elves as Thor (next to Odin) is the strongest of the Asgardians. During their battle, he proved himself stronger than Thor, being able to easily overpower him and knocking away Mjølnir with absolute ease.
Superhuman Speed: Algrim, like all Dark Elves is capable of running and moving at speeds much greater than those of even the finest human athlete.
Superhuman Durability: After becoming a Kursed, Algrim's body became incredibly resistant to all forms of physical injury. He was capable of withstanding powerful blows from Thor's hammer without sustaining injury.
Visible Evidence
IN THE CLIP THAT FOLLOWS: Frigga is held in a stranglehold by Kurse and the sword is plunged directly into her heart, killing her instantly. Kurse has no chance of missing, likely shearing her heart in half.
