Why was abolitionist John Brown executed?

Due to his role in the raid on Harper's Ferry.

John Brown organized a raid on a military arsenal in Harper's Ferry, Virginia.

John Brown was an extreme abolitionist who believed he was doing the work of God to free slaves. Brown organized a group of abolitionist and runaway slaves to raid the military arsenal in Harper's Ferry, Virginia. Brown was going to use the weapons to free slaves from area plantations. Due to the violent actions taken by Brown and his followers, he was executed.

John Brown was executed by the government because of his leading in the Harper's ferry raid.