Again Mr. pohnpei397 provides a tremendous backdrop to the response that you require. If I may I would like to delve a little more into the "attitude/s" or mindset that existed in the antebellum era by slaves. Of course this has been an area that received tremendous attention in the decade or so; perhaps the most enlightened of works on the topic was done by Kenneth M. Stamp in the book titled The Peculiar Institution.
In Stampp's work, The Peculiar Institution, he argues that on many a plantation throughout the American South their existed between master and slave a paternalistic relationship. What he found was that the interaction was much like that a apparent and child; because slaves had t know the fine line between a properly behaved chattel and one that was insubordinate (Stampp 329). Furthermore, many slaves, on many levels, could manipulate whites, etc. but if it were discivered that the manipulation were done in a deceitful way retribution was eminent. There did develop in some cases a quality, almost respectful, style of interaction or relationship between master and slave. Perhaps the best work to address this perspective is by Andrew Levy titled The First Emancipator. This work takes one through the life of a slave owner Robert Carter, who eventually emancipated all of his slaves. From this slaveholders life it will become overwhelming evident that on some plantations there did exist a genuine respect for one another. For example in the year(s) just after the American Revolution slaves actually escaped to Carter, not away from him (Carter. 116). In summary, it is important to understand that it would not have been unusual for some slaves to tell a a white person about the possibility of a slave revolt. This is due to the very "peculiar" nature of relationships that existed on varying plantations between slave and owner. This peculiarity could run so deep that not even one slave to another could understand if they were derived form different plantations that provided varying ways of conducting business.
There is, to my knowledge, no concrete evidence as to why the informants exposed the Denmark Vesey plot in 1822. However, it is possible to make informed guesses. I would argue that the betrayal shows us how different slaves experienced the system of slavery in different ways.
It is important to note that the plot was betrayed by a house slave. House slaves were often seen as being closer to their masters--having better relationships with them. Because of this, the plotters had tried to avoid recruiting any of them. However, someone eventually brought this house slave in on the plan and he told his mistress.
This shows us that different slaves had different attitudes. A slave who was well-treated might have a pretty positive attitude towards his masters. This might be especially true if he was a house slave interacting with his masters on a regular basis.
So I feel that the slave informants betrayed Vesey because they had sympathy for their masters and did not want to see them hurt. This shows us that not all slaves experienced slavery in the same way and did not react to being enslaved in the same way.