The film's new (Disney-canon) Junior novelisation "So You Want to be a Jedi?" strongly indicates that Luke was reluctant to kill the wampa, he merely wished to escape from it. Having wounded the beast and released himself, Luke left the cave in a somewhat foolhardy attempt to get back to Echo Base.
The wampa staggers back, staring. The lightsaber is so sharp, so hot, that it has cauterized the wound. There is no blood. But there is no arm either. The great beast is in pain. And now it is afraid of you. Very afraid.
Keeping your eyes trained on the savage ice beast, your lightsaber raised high, you slowly back out of the cave.
The earlier Junior novelisation gives us a bit more info about Luke's mental state. He was keen to evade the Wampa as well as being dazed and confused, evidently suffering the early stages of hypothermia and, presumably a low-level concussion
The lightsaber sliced through the ice, and Luke kept the weapon activated as he tumbled to the cave’s floor. He sprang to his feet just as the wampa was about to pounce, and swung the lightsaber hard. In a single motion, he cut off the monster’s right arm. The severed limb landed on the snow with a muffled thud. Howling in pain, the wampa clutched at its open wound.
Not wasting a precious second, Luke deactivated the lightsaber and scurried away from the wailing beast. He moved by instinct, pushing his way through snow and ice until he tumbled out through the mouth of the cave and into …
A blizzard.
When I wanted to leave Tatooine, I never bargained for this.
Dazed and lost, Luke pressed on, leaving the cave far behind as he moved deeper into the storm.
