What is this insect with scary antennae?
Actually, the antennae of that insect are not scary at all! 

In your image, the antennae are pointing down; those forceps-like structures pointing up are part of abdomen, and are called cerci.

That insect (probably a male) is an earwig, from the Order Dermaptera. In the image below, the antennae are at the left, while the abdomen and the cerci are at the right:



Narrowing down the the Genus of your specimen is more complicated, but here is a list of earwigs from Sri Lanka: https://en.wikipedia.org/wiki/List_of_Dermapterans_of_Sri_Lanka

Regarding the danger: earwigs are not dangerous. The cerci may be scary, but they normally are not capable of hurting a human. Also (and I hope you don't believe urban legends), they do not lay eggs in human ears: http://www.snopes.com/horrors/insects/bugear.asp