Hostage, a person held in captivity to assure that an agreement is observed or to coerce another party into taking (or not taking) some action. Countries in ancient and medieval times sometimes exchanged hostages, often persons of high rank, who could be put to death if certain agreements were broken. Invading armies often take hostages of local citizens to discourage resistance to their occupation. Terrorists sometimes seize hostages both to force concessions from governments and to gain publicity for their cause.
What makes a person not only murder, but murder multiple people over days, weeks and even years? There's a special name for these types of murderers: serial killers.
With modern security, how can thieves walk out of a museum with millions of dollars worth of art? It usually just takes force and a little bravado.