Sollozo was a made man in Sicily (he did two terms in Italy and kept omerta) and was simply trying to expand into the New York market. He went about it in an "honorable" way by approaching Barzini, having Barzini tell him he'd back him, but to use Tatt as his front man, and to get the blessing of Vito Corleone. When Corleone refused I am sure he went back to Barz and Tatt who wanted th drug money and got their permission to kill him. When that failed, it was all downhill for him from that point. The mere fact that the other families sided against the Corleones for the killing of Sol and
McCluskey showed their respect for Sol as one of their own.