Can you explain why killing Strephon was a deal-breaker? Monarchs get killed all the time. And big events open up story possibilities.
There was no lead-up. The Imperium seemed to be in a golden age of peace and plenty [absent some hints of corruption such as The Traveller Adventure exposed]. Then [literally] BLAM! Chaos!
It might would have all been more believable to me - and I don't doubt others as well - if there had been publication of a new campaign - perhaps "The Illish Plot?" - in which a group of player characters have a chance to find out about and stop the assassination. This campaign would have included - in it's synopsis and other information - all the relevant explanation about Dulinor's motivations that we got later, when it was rather too late. I recall that something of the sort was hinted at in the SJG TNS dispatches, to the effect that an Imperial investigation had "failed" to discover who had been behind the bombing of Dulinor's shuttle, on that fateful day at Capital.
If GDW has published such a campaign first and then a year later assassinated Strephon anyway, it would have probably gone down much better with everyone. All you needed to assume is that the PCs f****d it up, which is no big stretch of the imagination. :)
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