Planters feared slaves not so much because of the ill treatment slaves received at their hands, as the first answer above implies; that is a situation which could easily be remedied. Rather, they feared them because of the general fear of a slave rebellion in opposition to the institution itself. Additionally, in at least one colony, South Carolina, slaves outnumbered whites considerably. One of the major Slave rebellions of history occurred in South Carolina, the Stono Rebellion.
There was mistreatment of slaves unquestionably; but this was not universal; there was, however a universal resentment of the institution itself by slaves. There is no indication that the slaves of the Stono Rebellion were mistreated; rather they rose in rebellion to the institution, killing all whites in their path and raising a flag of freedom as they marched. The situation had been exacerbated by interference from the Spanish, who offered freedom to any slave who escaped and converted to Catholicism. Additionally, news of the successful slave rebellion in Haiti (the only successful slave rebellion) caused great consternation among whites, as they feared their own slave population might be inspired by the revolt. For that reason, they were often prohibited from learning to read and write or to congregate in large numbers. The following account of the Stono Rebellion is a good indication of the fury of rebellious slaves and the fear it engendered in the white population:
September 20th a Negro came to the General and told him that was said of the Negroes rising in Carolina was true and that they had marched to Stono Bridge, where they had murdered two storekeepers, cut their heads off and set them on the stairs, robbed the stores of what they wanted and went on killing what men, women and children they met, burning of houses and committing other outrages and that one hundred planters who had assembled themselves together pursued them and found them in an open field, where they were dancing, being most of them drunk with the liquors they found in the stores. As soon as they saw their masters, they all made off as fast as they could to a thicket of woods, excepting one Negro fellow who came up to his master. His master asked him if he wanted to kill him. The Negro answered he did, at the same time snapping a pistol at him but it missed fire and his master shot him through the head. About fifty of these villains attempted to go home but were taken by the planters who cut off their heads and set them up at every mile post they came to
An even greater fear was treachery on the part of the slaves individually. More than one master was murdered by a slave, quite often by poisoning, as the latter often retained knowledge of roots, berries and potions that could cure or kill. More than one Master met his maker when a slave poisoned his wine. Still others were killed by stabbing, blunt force blows, etc.
It is interesting to note how the fear of slave violence manifested itself in the responses of whites. No white person was ever burned at the stake in the colonies or new U.S.; however many slaves who murdered their masters where put to death this way as an example to others. A futher example was set by the slaves who participated in the Stono Rebellion. Their heads were placed on pikes on the roadside leading to Charleston as a warning to others who contemplated similar ideas.
Planters feared slaves because they had generally treated the slaves badly and were afraid the slaves might take revenge. This was made worse by the fact that most planters lived on plantations with very few other white people, surrounded by slaves.
With this in mind, imagine how you would feel in such a situation. Here are these hundreds of people that you own. Sometimes you have them beaten. You might even sell their spouses or their children away from them. Sometimes (if you are a man), you might force them to have sex with you. Now imagine that there are only maybe 10 people who are on your side on the entire plantation. This is why you would fear the slaves and want to take steps to ensure they could not possibly attack you.