Erato,
What a terrible terrible story. I Googled and found many references - including a photo of the gravestone of poor Arthur and Margaret. Not to mention sites which say that Louise Peete (said to be a refined and educated woman; they also say she died in the gas chamber) was actually a serial killer who had perpetrated half-a-dozen murders. A post like yours certainly stops one in one's tracks and makes one ponder along so many lines like prison, rehabilitation, human nature, good works, etc, etc. Though perhaps there is, at least, comfort in the fact that the murderer was not part of your own heritage.
I know that one must be prepared for whatever one finds on one's family tree - and sometimes I complain about my own oh-so-boring ancestors - but I'm sure there are people who sometimes wish they had never begun their research. Two cases I know (I hope I recall them correctly) are a person whose Ggpa (quite a famous professional in his own city and on day leave from a mental facility) murdered - with extraordinary violence - the person's Ggma; and another where a person traced his/her distant ancestor to the issue of a case of incest where it was the very young violated daughter who was blamed and imprisoned while the widowed father who was responsible went scot-free.
JAP
PS: Erato, what a violent connexion for a poster whose name is, as I recall, that of a gentle poetic muse and/or an inoffensive tiny gastropod.