I think both categories are relevant, for the different reasons you mention. An aircraft that is either unstable or over-challenging (Starfighter?) will kill pilots in any circumstances.
A 'safe' aircraft that is nevetheless unsuited for its combat purpose, is equally lethal.
There is a third category which includes aircraft having a generally sound flight envelope but which have a specific design flaw. Examples are the Meteor dive and Barracuda loss of elevator control. Neither stopped the aircraft going into service and it was education that stopped attrition rather than elimination of the problem.
Most truly dangerous aircraft would have been whittled out of the system during testing. Brickhistory is right in that poor training (F104) or unsuitable use of the aircraft (BE2, Defiant) resulted in a reputation many aircraft did not deserve. If your story covers any aircraft in this category I hope you would show that most went on to have safer careers.