What is toughness? What makes someone scary/tough? Is the guy who has killed 1000 people tougher than the guy who's killed one?

Let's pretend for a minute that there's a knight in the middle of town, his armor is so tough no one can defeat him. Knights come from miles around. Until the little old lady walks up and sticks a knitting needle in his neck, through a chink in the armor.

Space marines are not gods. They're just humans who can take more damage than normal. Not a lot more. A head shot will still kill them, a normal person with an autopistol can kill a space marine, if they get lucky enough. In game, normal humans save on a 6, and space marines save on a 5 or 6 (circa 20 years ago anyway). This makes them somewhere around X2 - X1.16 times 'tougher' than an unenhanced human. 

The most important part is, space marines are soldiers. They fight in pitched battles. If you have a million chaos cultists, they don't call inquisitors. They call space marines.

There's a reason inquisitors (and space marine chapters) don't get newly minted and march straight into the eye of terror to 'finish chaos once and for all'. Because both pick their battles, and only fight fights they think they can win. We don't go pick the highest PR mission we can find straight out of chargen. 

In a pitched battle, or a brawl, (chaos/) space marines will beat an inquisitor every time. In game, you are ignoring a million threats, to deal with the one you think you can deal with. And as an inquisitor, you stack the deck in your favor as much as possible. It's a surgical strike. A knitting needle in the neck of chaos. We are not combatants, we are murderers. We only fight when we 'know' we can win.

The space marine who has killed 1000 people and goes toe to toe with the knight in the unbreakable armor is 'tougher'...but he's dead and the little old lady isn't.

So anyway, don't think of space marines as being made of paper. Think of them as being set up, disadvantaged as much as was humanly possible, and then coldly assassinated. We're not 'fighting'. We're exterminating.

K.