From 1d4chan
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Lulz is a perversion of LOL, in that LOL is caused by anything, while lulz is caused by more evil types of humor, i.e. trolling, an heroes, tragedies, plot derailment and so forth. Lulz are exceedingly common on 4chan, if not life in general due to how much FAIL humanity can accumulate in a single hour.

"For the lulz" is a 4channer's justification for everything they ever do even if ESPECIALLY if the endeavor didn't actually produce any lulz and instead was motivated by pure spite for other sentient life or an obsession with ruining someone else's day.