In English, people often under- or overnegate and get the meaning across just fine, because no one really notices and it makes sense in context.
Anonymous
My feeling was something similar might be going on, where if you aren’t paying close attention you can read the sentence and the meaning is pretty clear, but if you look closely it starts to seem a bit off. What do you think?
As @Chocolate said 殺す would be better than 殺させる (where レムにLを殺すことを確約させた月 would translate to: 月 made レム promise that/to Lを殺すこと)
Here レム makes the promise
But after looking at this (dictionary.goo.ne.jp/word/en/commit definition 3) I guess レムにLを殺させることを確約させた月 can be translated to "月, who commited himself to make レム kill L