An animal rights group says a fancy dress costume of the hunting dentist, Doctor Walter Palmer, "trivialises" the killing of Cecil the lion.
The dentist killed the animal in Zimbabwe after paying to take part in a hunt leading to anger across the world.
The company behind the outfit says it "features a severed lion's head mask, bloody smock, and bloody gloves for a gruesome, complete look".
Born Free says Halloween promotes a fantasy around the situation.
The charity says that the reality for lions is that "they could go extinct in our lifetime".
The online shop is selling the costume for nearly £40 plus postage and packaging and claim it highlights "2015's most controversial killing".
Mark Jones from Born Free told Newsbeat that the costume is "rather an inappropriate way of depicting a very serious situation".
The company selling the outfit has promised to give 15% of the cash they make to a wildlife foundation, but Mark say he isn't impressed.
"While we applaud the idea that some of the proceeds should go to lion conservation, nevertheless it does seem inappropriate to be trivialising this very serious issue in this way.
"Halloween has become synonymous with gore and frightening things, and Born Free certainly finds lion hunting a gory blood sport with frightening consequences for lions.
"Across the globe lion populations have dwindled to less than half what they were 35 years ago."