But how is Hela worthy of it? She has evil intentions and wants to destroy Asgard.
Because when Odin died, the enchantment on the hammer no longer existed.
We first see Odin put the famous enchantment on Thor's hammer near the beginning of Thor..
At that time, Odin had striped Thor of all his power and imbued the hammer with it. From there, he added the enchantment to restrict Thor from being able to use it until he changed his ways of thinking and behaving (and, perhaps, anyone else that comes across the hammer.. but really it's Thor's hammer, so, Thor).
"Whosoever holds this hammer, if he be worthy, shall possess the power of Thor"
When Odin died though, his magic died along with him. This is, in fact, the very reason why Hela is able to return to Asgard. So, although they didn't make a point to show it, the enchantment did wear off with Odin's death.
We know that this enchantment lasted beyond Thor because it's referenced again during Avengers: Age of Ultron, when everyone takes turns trying to pick it up (with Captain America actually wiggling it). Also, there's that scene with Vision picking it up and everyone being both shocked and automatically trusting of him because of it.
At the same time, Hela is extremely powerful, and it's quite possible that she wouldn't have needed the enchantment off the hammer anyway, in order to stop/destroy it..
Lastly, it is confirmed that Hela held the hammer, instead of just stopping it. From the script, obtained from Walt Disney Studios Awards:
Thor winds up and HURLS Mjolnir at Hela...
-THUD- the mighty hammer comes to a cold stop, against Hela's hand.
Thor is shocked. He stretches out his hands, trying to call Mjolnir back to him.
The hammer trembles in Hela's hand, like a mini game of tug-o-war. But ultimately Hela keeps possession.
THOR: That's not possible.
HELA: Darling, you have no idea what's possible.
HELA SHATTERS THOR'S HAMMER.
This is also shown in the film, with Thor reaching out and there being a brief struggle before Hela shattering Mjolnir.
Top - After Thor throws Mjolnir, Hela catches it; Bottom - Thor tries to recall Moljnir but Hela is holding on and won't let go.