Let us not mention this to our space alien visitors ...

There's really no need to bring up this sort of thing ...

Damn Interesting: The Thugs of India

...Their extreme secrecy combined with their mastery of murder made the Thugs the deadliest secret society in all of history. In the early 19th century they were credited with 40,000 deaths annually, stretching back as far as anyone cared to count. Some estimates put the overall death toll as high as 2,000,000, but with the cult potentially operating for more than 500 years before formal records were kept, the true number is impossible to determine.