As Tris fled from the scene, she knew that she couldn't just leave Kazuichi alone. After all. He was severely overpowered by the Zombie Plant. If she didn't come to his rescue, he was as good as dead.
On the other hand, Tris was fairly aware that heshe couldn't just leave her younger sister alone. She was still in a trance thanks to the Zombie Plant. If she were to leave her alone, she would most likely walk towards the Zombie Plant once again.
And it's not like she could sacrifice the wet towel around her mouth and nose. She was going to help Kazuichi so she couldn't afford to get hypnotized by the Zombie Plant.
