Ali's voice becomes shrill when he remembers the exact moment when he decided to flee al-Shabaab in 2007.

"I spent one year with the Shabaab, training with them, fighting, assaulting villages,” he said. “Then one day we went to a village whose inhabitants did not want to pay us taxes. They were all massacred. At least forty children were killed. I couldn’t do it anymore. I saw all the blood, those dead children, and I hid and I started to cry. Why do the Shabaab not accept that their soldiers weep? Especially in the face of the dead. If they see your tears, they kill you. That day I decided to run away.”

Ali (a nickname he’s chosen for security reasons) is a 29-year-old Kenyan who was enlisted by al-Shabaab, a Somali militant group affiliated with al-Qaeda, in Kenya in 2005 and was sent to fight in Somalia. He doesn’t remember how many people he killed, but his eyes are bright with tears when he talks about attacks on villages, defenceless people being killed, children massacred. I met Ali on the roof of a building in the Muslim Quarter in Nairobi city that in recent years has suffered several terrorist attacks in which hundreds were killed.

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Ali, 29, was enlisted by al-Shabaab in Kenya in 2005 and sent to fight in Somalia. After a few years with the group, he regretted his decision and fled to became a volunteer. Now he works in Nairobi's slums with local NGOs trying to prevent that other street boys make the same mistake.

Ali, 29, was enlisted by al-Shabaab in Kenya in 2005 and sent to fight in Somalia. After a few years with the group, he regretted his decision and fled to became a volunteer. Now he works in Nairobi's slums with local NGOs trying to prevent that other street boys make the same mistake.

Ali, 29, was enlisted by al-Shabaab in Kenya in 2005 and sent to fight in Somalia. After a few years with the group, he regretted his decision and fled to become a volunteer. Now he works in Nairobi's slums with local NGOs trying to prevent that other street boys make the same mistake.