First Time RPG'r needs help fleshing out a back story

JerseyFrank went on a bit of a rant said
3

Game: Blades In The Dark
Setting: Duskwall - A distopian, Victorian-ish land where it is eternally dusk, and ghosts do bad things to the living. Ghosts rise from their mortal coils three days after death and gradually change from themselves to whatever bad thing suits the DM whenever necessary.

The Party: We are a band of petty criminals. Criminal gangs are common/numerous. We are but one. Gangs vary in strength, size, and with whom they are affiliated. We recently ran into an old "frenemy" of mine, Lord Scurlock. Scurlock and I were equals/peers at one point, but his power/clout has grown rapidly and tremendously. This encounter won't be our last.

My Character: Wellesley Grambingston - A "whisperer" (i.e., wizard) who escaped a military academy for "orphans" after being kidnapped at a young age. His personal drive is to be reunited with his forgotten family and exact revenge on those that are responsible for his abduction and training/torture.

Here's the part of the back story I need help with...
I escaped the academy with a small group of other coming-of-age "orphans." The circumstances of our imprisonment and common goal of escape led us to be friends. Scurlock was among them. Sometime during or after the escape, two of these friends died - Nyrryx and Scurlock.

Oh, and for some reason, I am the one who killed Scurlock.

Nyrryx lived on by possessing a prostitute.
Scurlock went on to be vampire, taking the name Lord Scurlock, and is a pretty big deal in this crime-ridden town.

How did they die? Why is Nyrryx still my friend, but Scurlock my bitter rival?

Knock yourselves out... I'm just looking for ideas to literally bring to the table.