2 Answers
While God did command nations to kill, it's important to remember that he never commanded individuals to kill devoid of their representing a nation.
So no, I don't think you could say that it is legit for any single individual to claim that God commanded them to kill. On the contrary, one of the ten commandments is, "You shall not murder." (Exodus 20:13). The word that God uses for murder is different than the word that is used to mean "kill" when it is carried out in war.
So to answer your question, yes. You can deny their claim. They were not told by God to murder.
You can read more about the words used
here.