Looking at it a bit from a pure MCU perspective:
-Odin states Hela draws her power from Asgard.
-The Odinforce in the MCU supplementary books state its a mystical force used by the Kings of Asgard.
-As the first-born to Odin, Hela arguably has a mystical claim to some or all of that power, and rightful heir to the throne.
I would argue that Hela to varying degrees of her existence has access to the mystical properties of Asgard. However, while Odin was alive he had the full force and focus of the Odin-force. Hela, even if able to use some of it, could never muster enough to outfight Odin while he was alive and had priority when drawing upon it.
With Odin's death, she was able to draw upon more and more of it, without anybody else like Odin countering that draw. Of course channeled through her, she became an unstoppable force of death (at least until Surtur shows up, who is specifically designed to counter the Odin-force.)
Basically with Odin gone, you could argue Hela is comparable to Odin (and slight lack of power would be offset from the apparent lack of need for the Odin-sleep.) As she drew more power, and learned to harness it in Asgard, eventually she could have left Asgard and been an unstoppable force not unlike Odin.
Thor doesn't have the power to directly kill her. No Asgardian could. She is a death goddess drawing power directly from an entire realm. Only Surtur, who has the power to basically cancel anything Asgardian could have defeated her (and of course Gauntlet Thanos.)