I mean, it's totally possible to die and get better. A cousin of mine had a nasty car crash a few years ago. Died twice. He's fine now. Granted, I don't think he's killed anybody so the point is moot for him, but people absolutely can have defined K/D ratios.
What do you mean exactly? Many videogames have a functionality where you are down and bleeding out or something for some time, in which you can be revived (not counting as death). After that, you have to respawn (counting as death). And real people can't be respawned, but revived, so the original take still stands.