I encountered a situation where my character died without any visible cause, and the Hall of Fallen Heroes marked him as killed by an Unknown Killer. This event made me feel frustrated and less inclined to continue playing Hardcore, as characters can die in any moment without any control over it. Additionally, the character died in a peaceful setting, during the change of waves in Infernal Hordes
Hello,
It happens sometimes.
It can happen even in town.
I guess it is caused by a moment when you should have died and it has bugged at that moment. Then your hero is a walking dead and it dies randomly.
I’m not sure about it and I can’t see no other reason to die in peaceful places or in town.
This isn’t the first time I’ve died like this, and I’m tired of dying for nothing. It’s hard to play like this.
Dead? 
Yes, it’s impossible.