As I recall in the novel Luca essentially says to Bruno that he'd never go against Don Corleone (to be expected, and something Bruno probably knew), but he indicates that he's looking for something to do on the side.

I think the only flaw with that plan was that Sollozzo and Tattaglia had planned to *kill* Don Corleone and so they *had* to get rid of Luca, not because they smelled a rat (with all due respect to Turnbull, I think it wasn't out of the ordinary for a man like Luca to want more money, and he was explicit about NOT being involved in anything that would go against the Corleones due to his respect for the Don - I seem to recall their discussion involved Luca being muscle only), but because they knew full well when they killed the Don (remember they didn't plan for their two guys to screw up), Luca would be after them out of revenge. Even if he was on their payroll for the drug racket, and the hit happened months or years down the road, he would turn on them in a heartbeat out of loyalty/respect to the then-late Don Corleone.

I'm pretty sure that the discussion to kill Luca must have been heavily debated, since having a guy like that on your payroll would be a great benefit, but Luca had to die to avoid repercussions for what was to come.