Yang Qing's Yin Yang jade bone physique made him sensitive to certain types of qi, especially ones related to slaughter, fiendish qi, or corpse qi. Yang Qing didn't detect a dense slaughter qi on Ma Yuan which meant he had fewer bodies on him.
If he could, Yang Qing wanted to help him with whatever pushed a man like him who seemed to be at the prime of his life, to the point he would willingly sentence himself to a painful death. From the earlier words that he spoke in a daze, he felt it had something to do with the person who was called Ruo.