Gotti is said to have rationalized the hit on Castellano as "kill or be killed." It could have been true. Supposedly the dinner at Sparks was to inform Gotti and others that Bilotti would be the new underboss, and that Gotti's crew would be dismantled. I doubt he would have been whacked at that meeting, but Gotti had good reason to infer he wouldn't live much longer.

I don't know if Castellano thought that Gotti and his crew were becoming so powerful that they were a physical threat to him--maybe, maybe not. But, their drug dealing was a threat to Castellano's defense in his upcoming RICO trial, and their loyalty to the departed Neil Dellacroce was a thorn in Big Paul's side.