Blood Atonement
Seems to be referring to the old ceremony, again maybe something they don't quite understand or do and are just causing problems? It seems they never consider that Christ's atonement was a blood sacrifice.
This is Discussed more HERE, but needless to say they are using it to say it was an excuse for early Mormons to rid themselves of Apostates. Actually the Blood Atonement is one of the more widely used attacks on the Church by Antis.