Even in Goodfellas, DeNiro says its always the close trusting friend that is ordered to betray the victim (Liotta). I think that's a general mob theme. It would be easiest for a trusting person to keep the victim at ease.


Regarding Tessio at the hospital, there is no indication that Tessio was a turncoat yet. In fact, it was at least a couple years later, after Michael takes charge that Tessio's disatisfaction becomes evident: "Pretty soon there won't be one place in Brooklyn I can hang my hat." He made the "smart move" by casting his lot with Barzini!


Also in the pre-cellphone, pager, etc. days, it was probably easier for the cops to arrest Tessios men without Sonny's knowledge.