Why did the Deuns of Dali pursue investigation of the Mo Yungs so aggressively?

[Ken Cheng]

In investigating the mysterious doings of the Mo Yung Family in DGSD, the forefront group that seemed to be leading the charge into uncovering the Mo Yungs' secret plans seemed to be the Deun Royal Family of Dali. King Deun Jing Ming, Prince Deun Jing Tsun, and their subordinates doggedly pursued the Mo Yungs. Shaolin, the Beggar's Union, and assorted other wulin entities and individuals had direct grievances with the Mo Yungs and were pursuing them as well, but seemingly without the focus and persistence of the Deuns. But why? Up until they began investigating the Mo Yungs, the Deuns had no grievances with them (and vice-versa). It was only after Mo Yung F'uk perhaps began feeling pressure from Deun Jing Tsun that he hatched that plot to conspire with the Four Great Evils to eliminate Deun Jing Tsun.

[Siven]

On a long shot, the Dali royal family wants good relations with the Song. So maybe they wanted to keep an eye on the Mu Rongs who are plotting a revolt.

My memory is hazy, but also didn't Mu Rong-whatshisname kill someone connected to someone else who was hiding as a retainer in Dali...

[Ken Cheng]

My memory is hazy, but also didn't Mu Rong-whatshisname kill someone connected to someone else who was hiding as a retainer in Dali...

Yes. In fact, it was Deun Yu's literature teacher. Although the guy had assumed the identity of a scholar, he was in fact a martial artist himself. The Deuns were good to him (as monarchs go, they were great bosses to work for...generous, respectful, and considerate), but it seemed to be quite an effort on behalf of someone who was essentially a servant.