I feel like the grading system is unfair and creates toxicity when you want chests

Now ive been trying to get an s rank in days for getting an chest, and here i really thought i was close(Im CatDogOFDOOM). I had really good stats, but i only got an A. The thing is, if i took all kills that i could( i gave my team some free kills) i know that i whould have gotten that S rank. Witch i feel is feeding the toxicity of some players, and is unfair. It makes the difference between taking all kills for a good grade, or making your team win. Therefore i feel like you should make assists be worth as much as kills, (scorevise not gold). To make ppl less toxic when they are strifing for chests. I managed got get their adc afk thx to me killing him alot early game and then i carried the game. I think that the leblanch said she got an s- in that game. Even through i got more kills, assists, died less, pushed more, communicated more and got more gold. HOW IS THAT FAIR?