Does Stun Stop Retaliate?

A just stunned opponent still has the muscle power to finish the retaliate. A just dead opponent loses all muscle control and the retaliate fizzles.

Does retaliate work if stunned?

Retaliate is explicitly categorized as an ability. A "Retaliate X" bonus ability grants a Retaliate X bonus. Stun can prevent the ability from occurring, but once the bonus is granted Stun does not affect the bonus.

Can you retaliate if you are disarmed?

If a monster is disarmed will it retaliate? Disarm prevents an attack, retaliate is not an attack. So they will still retaliate. Yes.