No, no. That's not what I'm gunning for guys - I want you to assume, to go in with the attitude that, your character won't make it out, even though they will! It's a mindset. As for your characters themselves, any persona is alright, so long as it makes sense for the person to be there, having joined the Marines. Like in real life, people that join the military come from all walks of life - some may be in there to get cash to support a family, others for the action and adventure, and others may even be some guy with a Bachelor of Science but can't find a job.
This plot won't be 'grim-dark' but it won't be completely gun-ho either. It will however, have tastes of both, and plenty of in-between.