I DO think that Preds have a code of honor, but, being they are aliens, it may seem warped and illogical to humans.
In general, the Preds seem to fight sort of unfairly, with their plasma caster and cloak, in order to flush out a real challenge; they won't go out of their way to kill something defenseless.
The whole self-destruct devise is was it is: a gadget that insures that there is nothing left of the Predator or any of its' weapons and gear.