The statements made in source # 10 are shown to be true in terms of the play in Act II, scene 1. This source basically states that Roman ancestors (Romans of the early republic) were more interested in the good of the republic than in personal glory; and that as Rome shifted from a republic to an empire, the powerful "contemporaries" that emerged were more eager to better their own power than that of the Roman empire.
This relates to this particular scene in that Brutus bases his resent for Caesar on the possibility that Caesar might continue this trend. He convinces himself (with some help from Cassius in an earlier scene) that Caesar is power hungry and that he will try to better his own image before he betters Rome’s image. Although Brutus has no evidence to validate his reasoning, because he knows about the gradual "moral degeneration" among Roman leaders, he believes that Caesar is more likely to follow that same direction than not. And so, because it motivated Brutus into killing Julius Caesar and thus shaped the rest of the play, the material presented in this source is vital to the course that the play takes.