It means they couldn't identify the exact cause of death, since there were multiple causes. For example, the victim may have been shot and also stabbed, and also received a severe blow to the head.
A PERSON THAT IS VIOLENT WITH TENDACY TO KILL ..
Not too sure, but I am REALLY interested in FBI, investigation stuff.
Homicide means murder, complex means complicated, basically complex, and violence, kinda easy.
So put it together, A very complex/complicated violent murder/homicide.
Hope this helps =]