a solid credibility creator would be to decide on a ratio and then creatively stick to it.... say 3 bad guys dead for every 1 good guy, now when setting your scenes all you have to do is decide how many bad guys you want to see killed to determine how many disposable good guys you'll be creatively killing off.
Terribly bad idea. Your´re just mass-adding redshirts to the story, completely defeating your own stated purpose.
That might even be worse than what you want to fix. Killing off disposable good guys doesn´t make much difference unless either director or story writer is a real wiz.
5 good guys have to die for a movie to have any real impact...... that's right for a movie to have an impact we the viewer cannot know who will live and who will die and that includes the really good guys.... a few of them have to go, sorry.... it's the difference between a great movie and a crap one, an epic movie and a typical piece of homogenous blandness.
No they don´t have to die, but the threat has to be credible, or perceived as such.
Though someone who is really good, can manage suspense and drama even without that just by good story, opposition or scene setting. Those tend to be very rare unfortunately.