[adiabatty]

Most (≈99.99%?) death threats aren't at all serious.

[ouid]

Every death threat is serious. Not many result in death, but that's not how we define serious is it?

More importantly, I can't come up with a reasonable argument that it's ever constructive to the conversation to be allowed to threaten someone.

[qb45]

Where I live "I'll kill you" is a common way to express dissatisfaction with someone's actions and while explicit, it's neither serious nor credible. I'm sure facebook is full of such posts.