@Eddnorn
Re-read the story from chapter 78 to now if you want to understand his motivations. To lay it out plain and simple, Sabin wants Yariel dead because her death will cause small dogs to no longer support the Tocqueville family who holds the power within Sabin and Eadies' home country, Thuringer, and cause Tocqueville family to fall out of power. Doing so will improve the living conditions for BOTH small and large dogs within the country.
As for Sabin's motivation for protecting Eadie, it's simply because he admires her for her strong sense of character, Eadie being a close childhood friend of his, and his own sense of selfishness which is shown in chapter 90, where he return to his home country to free his Mother from prison but his Mother refuses to leave, to uphold the dignity of their faimly.
TL;DR: Sabin's motivation = Helping his home country + Protecting a childhood friend he dearly admires.