Unfortunately, young guys can sometimes be pretty easily manipulated. I seriously doubt the 19-year-old was the mastermind. That would be extremely unusual. I think he was following his older brother and probably other older mentors. However, he still had the ability to say no or to go and report the whole plot to the police. So I’m not excusing him. He will pay for this with his entire life, and he should.
All of that said, if it hadn’t been for the influence of others, he might have stood a real chance to turn out a decent person instead of the total murdering scumbag he became.
No sympathy please. When you want to be sympathetic go look at the child he killed after standing right there and looking at him and his sister.