It's YABBA which fuels most of this sudden adrenaline-charged Jason Voorhees-style violence.
Meth. Speed. Crank.
The bargirls & even some of the Westerners use Ice but the Thai laborer's drug of choice is the old crazy pill.
They supplement it with whiskey which makes them even crazier.
But hey, his comedown is probably pretty awful. He is sobering with the realization he got high & cut off someone's head.
Any Westerner who ever lived cheap wwill tell you 50% of these rash bursts of crazy violence or homicides are meth-induced..