Tre is alluding to a fact that is being overlooked;
Double Homicide.
Either way, Amanda was most likely was an innocent bystander. She won't be the first, or the last, young woman to get mixed up with a possessive, psychotic loser.
The reality is that most Steroid dealers are often involved in the same World of crime and violence as any illegal drug dealers. In this case, he was involved with people with money, and he was in the verge of bringing some of these people down. A professional hit is not totally out of the question.
From what I've read, Amanda was smart, beautiful, talented young woman with a drive to be her best.
I am sure she will be missed by many.
Mufasa