Social contract checks out. Its a deadly campaign and PC death although rare is always a possibility. The game world is brutal and realistic. Not quite grimdark but there are powerful enemies afoot and the threat of death is always near when you have dangerous enemies.
Fair enough. I don't like PC death as a consequence, because death is not a consequence for the character (because they're dead, duh) - it is a consequence for the player. And I'm not here to make my players suffer and have to throw away a character they like.
So, consequences should affect the characters - which means they have to be alive to feel the weight of what they've done. Which means no dying unless people are monumentally stupid or willingly sacrifice themselves.
This might be unknown to the party and they might unintentionally aid the enemy.
"unintentionally" is in my opinion not something you want to use for major issues. If they release a major threat on the world, better for them to know it beforehand, or at least suspect it enough to consider it a significant risk. Having to choose between condemning their friend to an eternity of suffering or releasing a major demon or alien entity or evil god makes the choice *matter*. If they are unaware until after the deed, it's just you being petty
It's OK for minor things, just a reminder for players to check their six before doing something. But a major consequence ? I think it's much better if they know and choose to do it.