I don't understand how monotonously and tediously slaying things for the rest of eternity would be any less dull than a normal life. If you're really that bored, go skydiving.
The adrenaline rush of a real battle is far superior the kind of feeling you get from skydiving or any modern extreme for that matter. Life or death, kill or be killed. Bullets flying at you, tanks shaking the ground you walk, enemy's flanking from all angles, war cry's everywhere. You rely on training and instinct tell you what to do it almost becomes second nature, every battle is different, new planets, new tactics. You form a bond of brotherhood between your veteran comrades. Plus you have superhuman abilities, long life. Normal humans look at you in awe and respect and fear. If the lore and novels are any indication, a space marine is very hard to kill, you feel like a true god of war. Especially the ones who keep honing their martial skill every day, like what I'd be doing if I ever became one.
Gonna assume you've never fought before. Doesn't matter how many times you fight every fight is different and it never gets dull or monotonous. Sign me right the hell up, I'd be sacrificing a future family sure but the things I could accomplish in a thousand years or more outweighs that and the thought of progressing as a martial artist with that much training yeah I'm so up for that damn the risk.
You will be spending 8 hours at work another 8 hours sleeping, if lucky you will have 8 hours for your self, life goes in a circle round and round and is predictable in its outcome with different versions to it, today you will be helping perhaps a women with her finances, and tomorrow a married couple will be wanting help to take out a loan for house, the work is the same the version of it is the changing item, it is predictable. A space marine, going around slaying things, visiting entire new worlds, always new species or other human ones, being special and being admirred by the billions for keeping the humanity safe, having brothers among you leaves you never alone, you have recived a higher porpuse a direct meaning to your life, you know who you are, you know what you are to do, and you have a long lifespan to evolve in the many experiences you have had a chance to witness. In comparison you gain so much more in its fine simplicity, you are right of course if you only use tunnel vision and aim for that comparison.
But as much as you could achieve as a human, you'd still need to be alive to complete it. Think of all the lives saved and the good that has come of it because the Space Marines have fought and protected the race of man.
I'm not saying that Space Marines haven't done any good or anything of the sort, they've saved billions of lives in 40k but at the same time the amount of good a planetary governor or any other occupation within the governmental hierarchy of the Imperium can do outweighs what a Marine can do with his weapon in more than a few scenarios. And considering that in 40k most citizens are indoctrinated to think service to the Emperor in the most beneficial way possible is the duty of humanity and sometimes that requires people to refrain from going along the path of any militant branch whether that be the Astartes or the Astra Millitarum and so they must undertake an occupation that better serves Him and Mankind.
Actually i would choose something like a TechMarine, and i will not care about the Omnissiah the only thing i will do is, get the Humanity to the Stars!
