Does hogging all the kills screw your teammates out of an S rank? Maybe.
You are a YI that swoops in and goes 20 kills. Others on your team who were doing damage before your alpha get the assist but go 5 kills. If you leave them alone or distribute the kills everyone gets more gold overall than just you being fed and we get that sweet sweet chest. You were assured an S rank around the 10 kill. Stop KS'ing and starving your teammates. You are already fed and I didn't need your help anyway. YOU PROBABLY KNEW THAT THOUGH.
Edit: To those that think I am not focused on winning, do you think I am trying to get an S and lose?The only difference in me and you is, you want to win and I want to win in ROYAL FASHION!
Kill participation.