Why did Gao Yang let Andre follow? It was to show Andre how they fight, to show the power of Satan and let Andre know that cooperation with Satan is not him being taken advantage of, but sheer luck.
Everyone knows the power of a thermobaric bomb; it's a deadly weapon for confined spaces, but the problem is seeing is believing. Everyone knows the atomic bomb is powerful, but who would fear at the mention of atomic bomb?
In the end, knowing is one thing, but having never seen it, one won't have an intuitive feeling. The power of weapons is like this, as is the mercenary group. Everyone knows Satan is formidable, but just how formidable, and in what way, Andre does not know.
Actually, it's Gao Yang's first time witnessing the explosive power of a thermobaric bomb. He seemed calm, but the effects of the bomb shook him to his core.
