Is it ethical to steal a concept for a short story competetion?

A concept is a concept it actually depends on the way you put it, like maybe you tell a different story but with the same concept applied. So personally as a filmmaker I think it's not unethical to take a concept but make it yours with creative use of the concept, don't just take the video and submit it as a short film because that is unethical.

 

A malleable concept on it's own should be fine.

For example, I know there's a ton of "zombie apocalypse" media out there, they share the concept but tell the story in different ways. In a similar manner we have Star Wars, Star Fox, Star Trek, all in space with advanced tech, yet beyond similar title choices and theme, they're all quite notably different.

Just as long as you aren't tracing it word for word and porting/"recolouring" the characters without permission, I see no issue.