Who killed the hero Sigurd in the Völsunga saga?

In the Völsunga saga, Guthorm killed the hero Sigurd by stabbing him while he slept.

Guthorm was the younger brother of Gunnar and Högni, members of the Burgundian royal family. After Sigurd’s marriage to Guðrún and his deception of Brynhild, a bitter quarrel exposed the secret of Sigurd’s role in winning Brynhild for Gunnar. Brynhild demanded revenge, and Gunnar arranged for Guthorm to carry out the murder.

The saga says Guthorm was given animal flesh or otherwise driven into a violent state before entering Sigurd’s bedchamber. He stabbed Sigurd in the back, but the wounded hero remained powerful enough to throw his sword, Gram, after him. The weapon cut Guthorm in two. Guthorm also killed Sigurd’s young son, Sigmund, in the same episode.

Gunnar and Högni are common wrong answers because they authorize or participate in the plot, while Atli belongs to the later cycle involving Guðrún and the Burgundians. Details vary between the Völsunga saga and related Germanic traditions, but Guthorm is the canonical killer in this version.