In the name of science

Ah, yes, but you see, telling your subjects that they can die doesn't really have an effect. They may be scared, and quit, but the resolute ones will stay.

Other than testing on brave subjects, I don't really see how the results will change....unless you're thinking that the researchers will tell them what to expect in the experiment. But no, I'm thinking the researchers will tell them only what to do and what are the possible risks. How would that change the results?

Telling your subjects that they can die will cause the less resolute ones to quit. Therefore, you wouldn't be testing a representative sample, because you will prompt half the sample to quit, therefore, you'd be only testing 'selected' people.

And you don't think the results will change? In Zimbardo's experiment, it's possible that if only the braver people remained, they may not incorporate the conditions at all, rather, they might consider them above that and just sit in the corner and refuse everything. (After all, the guards were not supposed to harm the prisoners.) Thus, this would cause the participants to fail, because they are not fully participating in the experiment.

biases and skewing experiments are really an big issue.

It is. As a scientist, you don't want skewed results, you want representative results. Why would you only test something among a select group of people with similar personalities? It would be like (for a more extreme example), taking a poll on whether or not women enjoy sex, but then discarding all the results from the females.