This is how Lucil entered the Black Brotherhood, take three years later when he is twenty is when Bethseda's game begins. Lucil uses the Black Brotherhood to support his mother, she is happy he has found a stable job, but she doesn't know the job involves assassinations. He never told her. Lucil is rather polite and calm, he's quiet and gentle, cares deeply about life. He has conflicting emotions when it comes to his asassinations, in general when he kills someone of bad nature he feels that being an assassin could be a good thing, but when he kills someone he feels has already paid their societal dues he feels he must repent and redeem himself of this sin.
I do mention briefly that in the Dawnguard DLC with increase vampire attacks in Skyrim, Morthal was attacked. Lucil's mother was killed and this is why Lucil becomes a vampire hunter. At current Lucil is blinded by his grief, he's confused and is questioning his own morals and values. He no longers understands his path in life.
That's his backstory for now. Now unto the letter.
-------------------
For your eyes only,
Don't you find it weird that I am writing a letter to someone I barely know? I cannot even believe that I paid a courier to send you this letter. You are in the back of my mind, don't get the wrong idea. You are a beautiful woman and I am glad to have met you, however, this letter is not a confession of love. You are right now the object of my sorrow. When I stare upon you I am reminded of the things I lost.
But your philosophies and your views have my mind entangled. I do not know what is right and what is wrong any more. I had always though or believed to have thought I had balance in my life. It seems so silly that I am writing this letter to you, when you and I barely know each other. But you're the first person whom has seem to be interested in being directly involved with me. And directly involved in speaking with me. I do not know if I made the right choice. But I hadn't wanted to give up myself in that way.
I hope you understand. I do, however, want to speak with you again in person at some point in time. Thank you, I believe you have given me some things to think about at least.
~Lucil