Did you watch those movies?
What the protagonists are trying to do, and what the audience wants them to do, and what they usually end up doing is subverting the mission and delivering well deserved vengeance on the a-hole that sent them on it.
If I were setting up this campaign, subverting the compulsion is what I would expect would be the players primary goal and confronting and defeating the person who sent them on the mission as the self-evident climax of the campaign. As such, having several ways to escape the compulsion is a good idea.
You don't need a mechanical solution to a psychological problem
If the prospect of freedom is not enough to motivate them then the insta-death compulsion is likely to create a very real risk that if things get too tough, the players will just say "Why bother?", sit down and wait for the end. At least they get to die free, right?
Far better to think like this:
No man is an island, entire of itself; every man is a piece of the continent, a part of the main. ... any man's death diminishes me, because I am involved in mankind, and therefore never send to know for whom the bells tolls; it tolls for thee.
John Donne
"This is the mission, you have 48-hours or we kill your daughter."
Of course, I would still expect then to be trying to subvert the mission and get their daughter and f*^% over the guy who said this.
Playing by the Rules
NPCs have different rules than PCs - if you want your NPC to have the ability to hurt/kill the PCs from a distance just give it to them.
Options
Wish can do anything.
Interlocked Geas - start with the "prime directive", add another that compels them to attack anyone in the party who breaks their Geas, add one to any of the party who tries to prepare Remove Curse (ban Bards from knowing it in the campaign rules), etc.
