The watchman at Kanderendu shopping centre, reportedly caught the two after returning to his home on Sunday
Residents of Kanderendu village in Kigumo sub-county said he turned furious after his wife failed to give an explanation.
It is said he clobbered her and the man with a wooden bar and stabbed them with a dagger. His 12-year-old son, who witnessed the crime, fled through a window.
The boy sought help from neighbours who rushed to the home but it was too late. The bodies were found under the bed.
Kigumo OCPD Ibrahim said the suspect fled but that a search was launched and that he would be arrested.
"The woman worked at a tea farm and lived in her employer's servant's quarters with her family," he said.
"The two were from Turkana county. The identity of the dead man is yet to be established."
The police boss said a blunt object was used in the crime.
Mchumwa said they believed the suspect was in the vicinity as villagers reported seeing him at about 7am on Monday.
Nominated MCA Jane Muigai, a neighbor, said screams woke her.
Noting many murders result from infidelity, she advised faithfulness and amicable solutions to marital disputes.
"It is better to separate than commit murder," she said and asked well-wishers to help the boy as he does not have relatives in the area and needs counselling.
This comes only a week after a 19-year-old woman killed herself and her 30-year-old husband by setting their Maragua house on fire at night.
-ALICE WAITHERA