For the lulz
1. A term used to justify ridiculous, pointless and occasionally gratuitous behaviour.

2. Personal justification for the life of an internet troll.
Example 1:
Guy 1: Dude, why did you senselessly murder that guy with an axe while listening to Huey Lewis and the News?

Guy 2: Ah, you know, for the lulz.

Example 2:
Guy 1: So, you were playing World of Warcraft till 4:00 in the morning just so you could follow that one guy around and annoy him?

Guy 2: Yeah.

Guy 1: ...

Guy 2: For the lulz.

Guy 1: Get help.