The Clan Leader of the Snow Clan, Xue Xian's decision to go to the mortal realm had been approved by the elders of the clan. He had decided to come alone to look for Elder Xue Yan and find the people involved. This is the only way for them to survive.
After all, even the Starfield Sect made their move after giving them a future divination, and it was something frightening enough for them to cut off all connection with them. This shows that whatever the first elder of the Starfield Sect had seen was enough to frighten even a big sect like theirs.
Xue Xian brought his grandson, Xue Lengya, with him. After all, he had a hunch that this kid is involved with this issue one way or another. The one that was killed is his cousin by blood, and for him, who had no interaction with that mortal clan directly, he didn't know where to first start looking for the mortals that involve this problem.