克者何？能也。何能也？能杀也。何以不言杀？见段之有徒众也。 What does "defeat" mean here? It means "being capable". Capable of what? Capable of killing. Why not say "kill" directly? It is because Duan had his followers.
段，郑伯弟也。何以知其为弟也？杀世子、母弟目君，以其目君知其为弟也。Duan was the younger brother of Zheng Bo. How do we know he was the younger brother? When a crown prince or a younger brother of the monarch is killed, the one who does the killing is referred to as "the monarch". From this way of referring to the killer as "the monarch", we know that he was the younger brother.
段，弟也，而弗谓弟；公子也，而弗谓公子。贬之也。Duan was a younger brother, but he wasn't called "younger brother"; he was a prince, but he wasn't called "prince". This is to show disapproval.
段失子弟之道矣，贱段而甚郑伯也。何甚乎郑伯？甚郑伯之处心积虑成于杀也。Duan had deviated from the proper way of being a younger brother or a prince. By demeaning Duan, it also implies a severe criticism of Zheng Bo. Why is there such a severe criticism of Zheng Bo? It is because of Zheng Bo's long-cherished and carefully planned scheme that finally led to the killing.
