The sentence is as follows:
From the structure of the sentence this わけ doesn't appear to be a sentence ending particle, and I'm struggling to interpret it. I find that dictionaries unfortunately do not actually explain this word in a way that is meaningful for those who don't already understand how it is used.
To provide some context, the person currently speaking is watching the person that their 兄様 entrusted something to before their death, where they were also present at this time. The current time of the statement is as they are watching the person commit, for lack of a better word, war atrocities in order to do what they were asked.