He was so focused on killing Roth that he decided to smother him with a pillow. Wouldn't a quick and silent throat slashing have been more effective -- or if he knew he was on a suicide mission anyway, why not just shoot him a few times at close range? He must have been trained by the same Corleone ally who found the "twins" in Part III.
IMO, the fact that Roth was already in hospital, incapacitated with a stroke, gave Buscetta a perfect opportunity to murder Roth while making it look like a natural death. Since hit men are generally trained to stay "under the radar" and avoid calling unnecessary attention to themselves, I can't see Buscetta passing up an opportunity to smother Roth with a pillow, rather than slashing Roth's throat - unless for some reason Michael had wanted to send a particularly bloody message.
For that matter, there were even quicker and more effective methods available to Buscetta, had he been prepared to use them - such as injecting Roth with a poison or with a substance that would stop his heart, or introducing such a substance into Roth's IV drip. Since autopsies were not automatically conducted in 1958/59 in the absence of obvious evidence of foul play or other unnatural circumstances, there is every likelihood that those in Roth's circle would simply have assumed that the stroke had finished him off.
