They're called KDA whores, and you shouldn't blame them, you should pity them. Blame Riot for contributing to the influx of FPS morons after destroying any strategic depth this game had last season in favor of flashy explosions and 3 second teamfights. Anyway, I mute them and straight up tell them to go back to CoD where they belong since it's more "their speed." Wah wah toxic. If they're offended, they'll report me - don't care.
Back when Revive was in the game, I occasionally ran Revive TP Karthus, one, because it was hilarious, and two, because it worked. When you have someone who throws a hissy fit if you have 12 deaths and spends the rest of the game crying about your KDA, there's very little more satisfying than dragging them kicking and screaming over the finish line whether they want you to win or not. There's more to "getting gud" than having a high KDA - I respect someone with a bad score who knows what the hell they're doing more than someone who can go 7-0-5 and be bottom damage with no towers taken.
Now, if you have 12 deaths and have basically int fed the whole game, that's a different case, and it's clear to those who pay attention who's having a rough game and who's inting. Like a TF who is getting dove and targetted because card games trigger the enemy team or something isn't the same as the TF who flashes in place and ults in the middle of 5 people alone. KDA whores only see the second TF no matter the circumstance because they're not smart enough to know the difference or see what's going on.
I also want to point out that KDA is often simply K/D to them. Only kills and deaths matter for some reason, so the Nunu who is acting as the second support from the jungle is going to flamed at for being 2/8...I mean nevermind the 38 assists he has - those don't matter because again, they're not smart enough to understand anything beyond: "ME KILL! DURR YOU DIE! SAD! BAD TEAM! REPORTED!"