Hel is the goddess of death who manipulated Conrad Dippel to gain a vessel in the living world capable of holding her power. After Catherine Luss severed her brain's connection with her spinal cord, Hel's body was chopped up and burned and expected to potentially decay to nothing like the rest of Dippel's resurrected victims. The Winchesters were unsure if Hel would remain trapped in her cut-up vessel or if their actions would cause her spirit to return to her own realm of Niflheim.
Loki is a Trickster god and the son of Odin. After the death of his father, he blamed the archangelGabriel for it and with the help of his sons sold him to the demon Asmodeus. Years later, Gabriel killed him out of revenge.