Dying because you hold false beliefs that you didn't bother to verify is the epitome of stupidity. The fact that all people eventually die has no relevance to that fact. All people make some errors, and that doesn't change the fact that some errors are the result of stupidity and some are not.
Imagine a self-inflicted death that was as much a result of stupidity as possible, such as the creme de la creme of the Darwin awards.
Dying for a god is no less stupid than that. Dying for any cause that rests upon unverified beliefs is stupidity, where the level of stupidity corresponds to lack of rational support for the belief.
Who decides that?
I think you'll find that most people do not agree with your personal value assignments.
Stupidity is not a subjective value judgment, any more than height is.
Expanding on this a bit, I'm applying a standard definition that stupidity includes reaching a conclusion that has very low probability due to failure to apply reasoned thought, which fits quite well with the widely accepted definition of stupidity.
Beliefs that God commands you to do X, objectively meet this definition and that is not a matter of subjective values and more than whether evolution occurred is a matter of subjective values.
In addition, "stupidity" is commonly used to refer to actions that contradict one's own goals and intended outcomes. And it is precisely objectively wrong beliefs that cause this conflict with one's own goals. You wrongly believe that action X will lead to Y, when it will not or may even make Y less likely. If that wrong belief is the product of stupidity (failure to apply reasoned thought that would have avoided the error), then the action is one of stupidity. The theists goal to go to heaven is not "stupid", it IS a purely subjective value that cannot be correct or incorrect. However, their belief that they will go to heaven if they do X is almost certainly wrong and the result of a refusal to apply their own reasoning faculties. Thus, it is a stupid belief and any actions based upon it will not achieve the actors goal, and thus is stupid. In addition, their resulting death inherently contradicts and makes impossible ever other goal they had that entailed being alive. So, it is as stupid an action as any action can get.