If you believe [https://www.quora.com/What-is-the-ratio-of-combat-soldiers-t...], it varies from 7% to 14% of active military personnel that would be in ‘combat arms’, with everyone else being behind the lines in support roles. At least in the US. That lines up with my understanding anyway.
It’s actually a much lower percentage though. As most combat arms folks will be stuck guarding something and never see action, be in an indirect fire type role (artillery, sitting in a bomber or in a connex piloting a surveillance drone), or even if actually infantry have a super boring tour patrolling some pointless BS in the middle of nowwhere.
It also depends on where you draw the line at ‘killing’ of course. Does being a loader in an artillery battery count? A forward observer? A pilot who fires a cruise missle? A dude pushing the button to fire a cruise missle on a ship? An officer ordering an air strike?
About the only folks who are ‘killers’ in the way most folks would recognize the term though are infantry (of various forms, including mech. inf.), and perhaps some pilots if they have really good optics.
Soldiers follow orders. If they’re good ones anyway. Most have been trained to be able to be killers, and are often armed as such, just in case some folks need killing (per the chain of command and appropriate ROE).
But it rarely comes up, and anyone going into most MOS’s in the military hoping to do that is likely to be sorely disappointed.
And even in the worst wars, very few front line soldiers ever actually killed anyone - even infantry.