The guard allegedly stabbed his boss to death for “making him work long hours and verbally abusing him”.
Sawat Sriratchalao, 44, was arrested after being accused of pulling a 33cm knife from Arom Bannan, 56.
He is said to have been forced to carry out the attack in central Bangkok after being “intimidated” by his boss.
“I have been angry with my boss for a long time. Sometimes I couldn’t sleep because I was so stressed thinking about him,” he reportedly told police after his arrest.
“He bullied me and always spoke rudely to me. Even when I went home, his words were in my head.”
CCTV cameras are said to show Mr. Bannan begging for mercy in a parking lot in Lumpini Park on Feb. 1.
The assailant plunges a knife into his chest before bystanders intervene and separate them.
The suspect is believed to have escaped on his bicycle, with Mr Bannan falling to his knees.
The victim was taken to the hospital, but died from a stab wound to the left side of the chest.
Sriratchalao was detained after the police tracked him down at his home. He faces up to 20 years in prison if found guilty of murder.
Thailand retains the death penalty for murder, but it is used only sporadically.
An enraged worker, Sriratchalao, claims he was forced to work long hours and Bannan often endured verbal abuse.
Police Lieutenant Colonel Buddhawiro said: “Evidence shows that the suspect killed his superior with a knife.
“He planned the attack, so prosecutors filed a murder charge in this case.”
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