Shahidul Alam Jhinuk handed down the verdict after examining the records and 27 witnesses.
The condemned convicts are Sohel, 22, Rakibul Islam, Rafiqul Islam and Sultan Mahmud while the lifers are Rubel, 24, Sajib Khan and Akibul. Of them, Sultan and Sajib were tried in absentia.
The court also fined the convicts Tk 50,000 each.
According to the prosecution, Zahirul Islam, a class eight student and son of Khokon Mia of Khasherchar village, was found dead in the septic tank of Abdul Kaiyum on September 17, 2014, two days after he had gone missing.
The convicts abducted him and sliced him into five pieces following previous enmity.
Victim's father filed a case with Singair Police Station. Police submitted chargesheet against eight people on June, 2017.