If the Signless were thinking more clearly, he might have made his escape while the Initiate was distracted with his moirail. But he's not thinking clearly. In fact he's in enough shock that when the other troll pushes him aside, he just stands there dumbly, and it isn't until the Initiate addresses him with rage in his voice that he comes to himself enough to react.

He has to explain. He has to explain, because what he did wasn't wrong, it's okay, it was justified, it was right, and he has to convince himself of that as much as he has to convince the Initiate of it.

"No... no! You don't understand, he was dying already, he asked..."

It sounds so stupid. There's no way the Initiate will believe him.