Such sad news.. I dont' understand why they killed her and not just rob her... Its not like she could run to the cops about it.
They probably weren't concerned about Krista going to the cops; on the other hand, I'm sure they were scared that she would tell her boyfriend. He would have either gotten revenge on them when he was finished serving his sentence, or even more likely, he most likely has other drug associates who would have "taken care of business for him".
Drug addicts make dumb moves like this all of the time. The need for the drug overrides their ability to look and/or plan ahead. Their priority in life is acquiring drugs. I've seen dozens of young people steal from their own family and friends just to get their next high.