If you, like me, are interested in traditional diagramming (not the tree diagrams that are used at the university), you may want to explore the Reed-Kellogg diagramming system, which was used in many American secondary schools until World War II.
I am NOT an expert, but I am a big fan of Reed-Kellogg. Most American teachers no longer use it, and they probably do not know it. If they took linguistic classes at the university, they learned tree diagrams.
If you will carefully look at your sentence, you will notice the "skeleton" of the sentence consists of four words: thought made men sad.
All the other words modify (are attached to) those words.
In other words: (The) thought (of slavery) made men (like William Wilberforce) sad.
I am computer illiterate, so I do not know how to post the diagram here. Sorry.
Thanks for making my day with your fascinating question.