1.
If one argument or fact is set against another, it is considered in relation to it.
These are relatively small points when set against her expertise on so many other issues...
PHRASAL VERB : be V-ed P n , also V n P n
2.
To set one person against another means to cause them to become enemies or rivals.
The case has set neighbour against neighbour in the village.
PHRASAL VERB : V n P n