No easy answer, criminals by definition think they can better themselves easier by illegal means including drugs, prostitution, property theft, etc, a lot of which leads to violence.  Sadly, this case could be everything from a random encounter gone bad, a serial or gang killing, or to the likelier scenario where they knew one another in some fashion.                                Separately, we have the flood of cheaper and deadlier drugs into our communities, many of which make a sane person into Jekyll and Hyde with the resultant possibility to commit crime at all levels.  The age old riddle is why society is drawn to such, is it simple boredom, job / wealth hopelessness, the lack of good family, or what exactly?  Regardless, sympathies for all the victims and their families, just disgusting what's happening here of late especially when our greenbelts become danger zones.  RIP.