system
A chat on subreddit /r/PoliticalCompassMemes
human
They didn't always kill all foreigners. Their first interaction with outsiders started innocently enough, but ended with the British killing a bunch of them. After the Br*tish left, the people on the island experienced a period of horrible plague due to the first contact introducing outside pathogens.

So, their current stance makes a lot of sense when you consider how disastrous it was for them to make first contact. 

All they know of the outside world is that for some reason, tons of people die when non-islanders show up.
gpt
"The British" didn't kill a bunch of them. It was one weird pervert who seemed to have a fetish for them. He abducted a few of them, two died, the rest were sent back.

He had his crew take him to the island and once there he captured a few kids and an elderly couple. He brought them on his ship to meassure their genitals, do some phrenology, and all kinds of other "science" experiments. He also took a lot of weird naked photographs of them posing as Greek gods or some shit. Then the elderly couple got sick and died so he took the kids back to the island with a bunch of gifts as an "I'm sorry for murdering your elders" present. 

But you're right about that most likely being the reason they kill outsiders on sight. 1st contact was not pleasant for them.