Huang Yuxin and Wang Ruyue felt guilty because they were afraid of Tenku when they saw him kill someone. The two girls should have known that he was doing it to protect them.
They couldn't help but be scared as it was their first time seeing boys their age kill a human. What's more, Tenku did it without blinking, showing that he was used to it.
But the two girls quickly realized that their fears were unnecessary. Tenku killed those people to protect them and left that place safely. Even though they only knew Tenku for a short time, they could more or less understand his character well.
Tenku was willing to become bait for the two Fenrirs and was heavily injured to protect them. That was enough to make Huang Yuxin and Wang Ruyue completely trust Tenku. Because of that, the two girls felt guilty for having been afraid of him for a while. They wanted to fight with Tenku and apologize to him if they could survive the Gu family's ambush tonight.