Why don't the bad guys stop?

Question

In the 2007 film Shoot Em Up, a group of assassins under the direction of Paul Giamatti are attempting to kill Clive Owen, a pregnant woman he seeks to protect, and the baby she delivers.

Owen discovers that the baby's father is a powerful politician who had bred the child to serve as a source of replacement bone marrow, and infers that Paul Giamatti and his goons seek to interfere because the politician, who is suffering from cancer, will support gun-control measures if elected president, and they prefer that he dies.

It is eventually revealed that the politician is secretly in league with the gun lobby, rendering the initial assassination attempts needless. However, the politician is soon shot to death, at which point Giamatti and company continue to try to murder Clive Owen and the baby, even torturing Owen to try to get the location of the baby.

Why? Why at this point do they care? Once the politician is on their side, they don't need to kill anybody. Once he's dead, doubly so.

Answer 1

Two reasons: Hertz wants revenge and other plotters are still alive

The most compelling reason why Hertz (the hitman) wants to kill Smith even after the plot unravels is because his professional reputation has been trashed by someone who appears to be an amateur. Hertz is supposed to be a genius (constantly making smarts remarks about how clever he is) and is repeatedly frustrated by Smith with many of his men being killed. Even when he finds out that Smith has a background involving gun skills (and a tragedy that turned him into a drifter) he still seems to be motivated by a desire to prove he is the smarter, better killer. From the things he says and the way he acts, I suspect he would have continued to pursue Smith purely out of professional pride.

But there is also a reinforcing commercial incentive for Hertz and Hammerson. Hammerson (the gun company owner) is still alive and has offered Hertz a new, bigger, contract. Though their plot to infiltrate the presidency is over as the potential president is dead, they could still be prosecuted for the attempt as they have committed illegal acts for which the remaining baby is compelling evidence. So Hertz has an ongoing contract to protect and Hammerson has a whole business that could be disrupted by any revelations. And he could be prosecuted for serous crimes if any evidence of them ever appeared. Smith is still a threat as is the existence of the baby.

That's two compelling reasons why they still want to find Smith and the baby. So they are not going to stop. And Hertz still has his professional reputation to preserve which motivated him even more.

Answer 2

Hertz had no reason to not suspect Donna of knowledge of the Oliver purpose. And a witness with "some" knowledge, a proof in a form of a baby would be quite problematic for his employer. You know all these government "deal and no deal" when you are in prison for treason and/or attempted murder of senator/president candidate.
Hertz is also shown as a very strict professional. He could even let it go when Smith kills his employer. But the order is order.