Saying "murder and slavery are wrong" doesn't necessarily mean a belief in moral objectivism. Anyone using the term "murder" in that sense is clearly not communicating effectively. Murder is presupposed to be wrong or unjustified homicide, distinct from suicide, or justified homicide.Most people I talk to agree that murder and slavery are wrong, so apparently some people share it. Sorry if my choice of words is counter-intuitive though.
I think many people are just generally poor with the linguistic nuances of even fairly basic philosophy. Many people I come across seem to sway towards moral relativism when asked in a way less influenced by tricky language.