All the forums between here and reddit and gamer forums are pretty split; here is the sitch, looking for an official ruling.

So in the Flyboys Invitational yesterday this happened

Darth Maul (ps 5) killed Dengar (ps6)
Dengar wanted to shoot back with his pilot ability Fire Back.

Is he allowed to do this?
Does simultaneous fire only apply to ships with matching initiatives?
Can you do a retaliatory (or bonus) attack after you have died?

dengar can perform his bonus attack, yes. bonus attacks that are triggered by attacks are resolved during the aftermath step of the attack that triggers them. dengar is not removed from the game until all of the ships at the current initiative have engaged. even if maul is the only initiative 5 ship on the board, his attack still has an aftermath step.

this is described in detail under Attack starting on page 4 and continuing on page 5 of the rules reference. there is also a section on bonus attacks on page 6, a section on destroying ships on page 9 and on the engagement phase on page 11. the simultaneous fire rule (page 18) is relevant for this interaction, even though dengar and maul have different initiative values.

Excellent response, thank you for the references to the rules as well.

This isnt needed, as Meffo answered, but ill plop both relevant text here just for easy reference.

Dengar's Ability : After you defend , if the attacker is in your [front arc], you may spend 1 [charge] to perform a bonus attack against the attacker.

Which triggers here: c. Resolve any of the defending player’s abilities that trigger “ after you defend ” that grant a bonus attack.

This wasn't part of the question but I'll leave it here as a friendly reminder to whoever finds this that each ship is only allowed one bonus attack per round. For example, Dengar would NOT be allowed to take his revenge shot against Maul if he had already taken a bonus attack during the round such as with Bossk[Gunner].