They initially believed that Han Sen might have wings and was planning to fly into the valley to draw the snake king out.
It was nearly impossible for someone to walk down there. The depth of the snow was greater than any man, and if you were to sink into it when trying to walk on top of it, you'd be sinking into a white grave occupied by unwelcoming snakes. It would be certain death.
If Han Sen could fly, he would fare better, but not by much. The ice snakes also possessed the ability to fly, and so an unbalanced dogfight would quickly ensue. Although they couldn't fly too high, for as long as he would be in the ice valley, it would be impossible for him to escape their pursuit.
That's why they thought Han Sen's decision to go in, alone, was a suicidal move.
But there he went, walking into the valley. Without wings.