Sachin, 19, was having an affair with victim's mother Sana Khan and eliminated the boy identified as Sahil so that she could spend maximum time with him, according to police.
"Sachin was having an affair with Sana who along with her son stayed in his house on rent.
"She used to go to Moradabad, Uttar Pradesh to meet her daughter. Sachin did not like her frequent trips and wanted her to stay in Delhi all the time," said a police official.
"Sachin thought that if he will eliminate Sahil, Sana will bring her daughter to Delhi and not go to Moradabad again. This prompted him to kill the boy," the official said.
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As per his plan, Sachin abducted Sahil on Saturday evening and first banged his head into a wall, when he did not die of this, he strangled him to death and stuffed his body in the cooler.
Sahil's body was discovered yesterday morning by a neighbour on the first floor of a nearby building, which also houses a temple.
Sana had lodged a complaint with police. His father Javed, a tailor, lives separately as the couple had broken off several years ago.