Answer by taking a quote from the following article:

Osceola was named Billy Powell at birth in 1804 in the Creek village of Talisi. now known as Tallassee, Alabama, in current Elmore County. "The people in the town of Tallassee...were mixed-blood Native American/English/Irish/Scottish, and some were black. Billy was all of these."

Through the 1820s and the turn of the decade, American settlers kept up pressure on the US government to remove the Seminole from Florida to make way for their desired agricultural development. In 1832, a few Seminole chiefs signed the Treaty of Payne's Landing, by which they agreed to give up their Florida lands in exchange for lands west of the Mississippi River in Indian Territory. According to legend, Osceola stabbed the treaty with his knife, although there are no contemporary reports of this.  Five of the most important Seminole chiefs, including Micanopy of the Alachua Seminole, did not agree to removal. In retaliation, the US Indian agent, Wiley Thompson, declared that those chiefs were deposed from their positions. As US relations with the Seminole deteriorated, Thompson forbade the sale of guns and ammunition to them. Osceola, a young warrior rising to prominence, resented this ban. He felt it equated the Seminole with slaves, who were forbidden to carry arms.  Thompson considered Osceola to be a friend and gave him a rifle. Osceola had a habit of barging into Thompson's office and shouting complaints at him. On one occasion Osceola quarreled with Thompson who had the warrior locked up at Fort King for a two nights until he agreed to be more respectful. In order to secure his release, Osceola agreed to sign the Treaty of Payne's Landing and to bring his followers into the fort. After his humiliating imprisonment, Osceola secretly prepared vengeance against Thompson.  On December 28, 1835, Osceola, with the same rifle Thompson gave him, killed Wiley Thompson. Osceola and his followers shot six others outside Fort King, while another group of Seminole ambushed and killed a column of US Army, over 100 troops, marching from Fort Brooke to Fort King, in what Americans called the Dade Massacre. These nearly simultaneous attacks began the Second Seminole War.

So this settled it then?
December 28, 1835, Osceola, with the same rifle Thompson gave him, killed Wiley Thompson.