Some, and Washington was the most prominent example, felt trapped by slavery. In the late 1750s, he came to the conclusion that slavery was not a paying proposition, especially as it involved the monoculture of tobacco (and later, of course, cotton--to feed the greed of English mill owners) which destroyed the soil. He diversified his crops, hired many white workers (with whom he was about as displeased as he was with lazy blacks) and paid all of his workers, including the blacks, so that they would have an incentive.
But he had a problem. If he manumitted his slaves, they would be adrift in a world in which they were not prepared to make their way, and they would be a prey to unscrupulous men, and very likely would be spirited away into slavery again. Furthermore, half or more of the slaves at Mount Vernon came when he married Martha Dandridge Custis, and neither he nor she owned the slaves--they were held in trust for the two surviving children of Martha's previous marriage to the late Daniel Parke Custis. So, although many men might simply have ignored this, and dealt with the property Martha brought to the marriage as their own, which was the custom of the day, George carefully husbanded the resources of that legacy for his stepchildren. Of course, within a few years, some of the slaves he had inherited from his half-brother Lawrence Washington when he inherited Mount Vernon had "married" former Custis slaves, and to free some, while unable to free others, would have broken up families, apart from all the other resentments which it would have entailed. It was illegal for him to educate his slaves, so that was not a recourse for him.
Furthermore, Washington's quandary was enhanced when both "Patsy" (Martha Parke Custis) and her brother John Parke Custis died. This left a minor child as heir to most of the estates, and the rest the property of his wife Martha. If he made manumission conditional upon the decease of the estate owners, then some slaves would remain in slavery, the property of his stepgrandson George Washington Parke Custis, who had been born the same year his father died, and was literally a babe in arms at the time issue arose, while others would not only be manumitted (his slaves), but others would be awaiting the death of Martha to be freed. He didn't want to give the remaining slaves a motive for the murder of Martha, which could easily have been accomplished by poisoning.
At the time of the death of "Jacky" Custis, he left an estate of more than 18,000 acres and almost 300 slaves to his infant son. On his own account, Washington had about 300 more slaves. His eventual solution was to "retire" those slaves who were unwilling to work for the modest wage he offered, and to pay pensions to them all out of his personal estate. Those pensions were paid well into the 1830s, more than 30 years after Washington's death.
For the details of this dilemma and its solution, see George Washington: A Biography
(published posthumously), Douglas Southall Freeman, seven volumes, Scribner, New York, 1948-1957; and, Anguish and Farewell
, the last of four volumes of the life of Washington, (James) Thomas Flexner, Little Brown, New York, 1972. Both Freeman's entire biography, and the final volume of Flexner's biography were Pulitzer prize winners.
Others of the founders had fewer qualms, apparently, such as Thomas Jefferson, who neither manumitted nor made any special provisions for his slaves. However, as Frank has pointed out, not only were not all of the signers slave owners, i'll go further and point out that the majority were not