The Innocent by David Baldacci (2012) Grand Central Publishing

The first in a series of books about Will Robie, a hit man employed by the government.  It starts by showing us two of his hits; a gangster eliminated, along with his bodyguards, in Scotland, and an anti-western Saudi who is an embarrassment to his country.

Then Robie is given a hit in the U.S. which is not normal.  When he approaches the woman in her bedroom, he sees she is holding a child in her arms.  He decides not to kill her, but a back-up sniper puts a round through the child into her heart.  Robie snaps up the other child, in its car-seat and leaves it with a neighbor.  The second story line involves a girl in foster care who returns to her parents expecting to leave the D.C. area with them, but they are killed.

Robie and the girl get on the same bus out of town, along with a killer who wants to eliminate the girl.  Robie prevents the kill and they leave the bus, and it blows-up.

The problems become intertwined and Robie must solve who is trying to kill him and the girl.