Under certain conditions prisoners act violently against each other. In addition to many forms of intimidation, forced homosexuality, and general harassment, this may include knifing and murder. Sex offenders are especially vulnerable.
In the USA the prisons themselves are so rife with crime, feuds and violence that about 8% require protective custody for fear of retribution for sex offences (some committed against other prisoners), for fear of homosexual attacks, because of unpaid debts amongst prisoners. In 1980, rioting prisoners in the Santa Fe penitentiary used blow torches and knives to slaughter (and in some cases decapitate) 36 other inmates.