Robbers/burglars are generally sadistic and if you couple that with the isolation that a farm affords, which gives a perpetrator time, then you would get instances of torture etc.

However, there is some degree of animosity towards white farmers in particular, probably due in some part to the way in which they treat black workers which is well documented and has been for decades. If you read Sol Plaatjie's book 'Native Life in South Africa' (it's free online on Project Guttenberg and was published in the 1916) in which he details the unjust manner in which farmland was taken from Native South Africans (through the natives land act) as well as the barbaric manner in which workers were treated thereafter, you'll probably get a sense of where the animosity stems. That speaks to the nature of SOME of the killings and isnt a bigger factor than the burglars just having time but overall, there's no evidence of a 'white genocide' or a larger number of killings than anywhere else.