The Korsak Brothers Series by Rob Thurman

What makes us human? What makes us unique? And what makes us kill? Ten years ago, Stefan Korsak's younger brother was kidnapped. Now a rising figure in the Russian mafia, Stefan has finally found Lukas. But Lukas is now a trained, genetically-altered killer who his creators want back. Saving Lukas may be easier than surviving him.

Stefan Korsak and his genetically-altered brother have evaded the Institute for three years. When they learn the new location of the secret lab, they plan to break in and save the remaining children there. But one of the little ones doesn't want to leave. She wants to kill...