That's an odd complaint that actually makes Solid Snake sound even cooler
I think it has the best execution of everything in the entire franchise.
In my experience, I have never met a single person, not one, who has actually read a Superman comic, or actually
knows the Superman character for that matter, and dislikes him or thinks he's boring, one dimensional, etc etc the only people that say such things (again, in my experience) are people who don't know Superman, the people who are only familiar with the "aura" of the archetype Superman "represents" and has been passed on by the mainstream, but not know Superman himself.
Superman is by no means one dimensional, and in fact, when you put him with the rest of the league, Superman is the one who sticks to the truth and justice stuff the most making it a staple that the rest of the characters follow as an example.
Dude, read The adventures of Superman #40 and #41, and tell me he's one dimensional, I dare you
and there are plenty more examples.
I actually like the ending, because it's a big **** you to the people who wanted Snake to pull the trigger, fans included, Snake's a fighter, he's not giving up.