Consider, Tamerlan’s brother may have been turning 18 sometime in 2011 and a drug arrest while hanging out with his best friend would no longer be a juvenile arrest. Higher stakes. The druggie friend put Tamerlan’s brother’s citizenship at risk, while Tamerlan’s future citizenship was already at great risk after the domestic violence charge and FBI questioning. Note that Tamerlan couldn’t legally purchase-possess even a long gun. If Tamerlan’s brother were to get a drug arrest hanging with his best freind after turning 18 in 2011-ish, then tamerlan’s ability to stay, plan, and amass long guns under his brothers name in the future was at risk. Once Tamerlan’s brother got the citizenship, his brother could readily purchase and legally possess long guns. This may have been seen as essential to Tamerlan’s planning. The druggie friend had to go.
Further, if they wanted to train with guns and it’s likely they did, Tamerlan needed his brother to have citizenship. Else, how could they even risk traveling with the guns to go train with them. Tamerlan’s brother getting citizenship was essential to Tamerlan’s plan.
Tamerlan’s brother getting pitched with the druggie friend put everything at risk.