Capo,
I think the reason you feel that way is because Carlo was not a main character and we didn't know him as well as the others. However for the reasons TB mentioned, (especially the beating by Sonny), that that would be Carlo's main motivation for betrayal.
He knew he didn't fit in and would never be a part of the family really!!! I think, even tho we know little of him, that Carlo would haven given his eye teeth to become a "real" part of the Corleone family, but just didn't have what it takes.

I thought it had a great dramatic affect.
TIS