Depends on what country you live in and the same for the offender. In the USA, death threats of any sort sent through either electronic, voice, or written medium are strictly illegal and can lead to severe penalties, fines, law suits, etc., whether or not they are credible. I'm almost certain that making death threats in Second Life voids your user contract. Linden Labs SHOULD ban the offender. Keep trying to report this to LL. Look up your user contract for Second Life. There is a list somewhere of what can get you banned forever. I'm pretty sure that this is one of the offenses. Quote this to LL next time you report it.
On a more personal note, start hanging with a better set of people. If you are getting death threats, then you are hanging out with the wrong crowd. People who deliver death threats are either jerks, psychos, or cowards who want to bully you because they are protected by their anonymity in Second Life. Why would you want to expose yourself to those people? Would you do that in RL?