Salem might just be the best manga of all Epic of Remnants manga. The changes I've seen in this manga so far is really good and really enrich the story in this arc.
Its impossible for Sanson to protect three of them (Hopkins, Abby, and Lavinia). Its one of those situations in this arc again where you have to make sacrifice between the choices you have to pick just like Gudako in few chapters ago.
Sanson could either:
1. Protect Hopkins and Lavinia, Kill Abby.
2. Protect Hopkins and Abby, Kill Lavinia.
3. Protect Abby and Lavinia, Let Hopkins die.
I guess he gambled on the 3rd choice and also because of his moral conscience.