My impression was always that they planned from the start to kill Luca, and the very fact that Don Corleone felt that it would be believable that Luca Brasi, of all people, would be unhappy with the Corleone family shows that "the Don was slipping". I mean it was common knowledge that Luca was fiercely loyal to the Corleones, and would never betray Don Corleone. So who would believe that he was unhappy? Even if you reason that he may have figured Don Corleone was wrong to not go for the drug deal and wanted to get more money himself, this is Luca Brasi we're talking about. If Don Corleone didn't want the family involved in drugs, Luca would be too loyal to even consider looking for something on the side; he would have considered it disrespectful to Don Corleone.

So by Luca trying to act like he wasn't happy it was a clear sign that it was a trick and he was just fishing for information. I'd guess the minute he reached out to Bruno and Sollozzo (IIRC it was via some girl he was shacking up with who was one of Tattaglia's girls) they would have known it was a ruse and made plans to get rid of him. Anyone else might have been believable but not Luca. Even Clemenza or Tessio would probably have been more believable than Luca. Hell, I'd guess that even Sonny would have been more believable after his outburst! But Luca? No way.