Make Myself Just One More — Dean Wesley Smith

Mary Jo kills people for money. After a thousand years, she knows patience and skill and how to cover her tracks.
Mary Jo loves her job. She makes a lot of money as a hired assassin.
She also loves vodka and orange juice. Passionately, but not in a dangerous way. After a job well done, she rewards herself with the drink.
Mary Jo might be the coldest killer in all of fiction. Or at least the only really cold killer who loves vodka and orange juice.