I'm A Banana
Potassium-Rich
Eh setting up an encounter to kill a pc without any interaction from them doesn't sound reasonable. I hear some dms saying they applaud KM's decision. I wonder if the players in your campaign are as happy as you think they are. I guess I didn't realize that the PC vs. DM mentality was so rampant.
My guess is that there is a difference of philosophy here, of course. But it's not at all as sinister as the adversarial relationship you posit.
One of the functions of the DM, indeed, one of his *primary* roles, is to provide challenges for the PC's to overcome. This isn't any more adversarial than presenting a villain or a hostile orc with a pie. The only category in which this is different than an orc with a pie is in the hidden fatality of it.
The assassin, even in the killing of a PC, is just a challenge. Especially considering how easy the resurrection would have been. He's a roadblock to overcome. And in the overcoming, they are sapped of rescources, which builds drama and makes them feel heroic when they overcome the challenge. Thus, you have the essence of the game, the potential for failure and the ability to succeed.
Death, in D&D (and IMC) is just one more difficulty to overcome, one more orc with a pie, one more DC to beat with a die roll. It's slightly more annoying than hp damage.
This assassin's attack is no different, in DMing strategy, from including a mimic in a treasure hoard, or including a cursed item, or having a surprise round, or having a trapped door. There are, despite your denial, ways to overcome this. These ways would have required the PC's to be paranoid and cautious about what they were doing (as would be fitting fugitives from an evil empire) If they were, they might have avoided a death. Because they weren't, the enemy took advantage of the situation.