Just as G-d's presence fills the world but G-d is not the world, so too, G-d's mercy fills creation but G-d can't be defined by His attribute of mercy.
Some commandments, such as shiluach haken - the chasing away of the mother bird before taking her eggs or chicks - are expressions of G-d's mercy.
Other commandments, such as the commandment to utterly destroy Amalek, come to show us that mercy alone cannot sustain the world.
The revelation of G-d's mercy alone cannot define our commitment to perform His commandments.