I hate low elo (Zac with frozen heart:More inside)

I had a Zac in my game, who got a frozen heart over a Randuins so that Vayne would do less damage to him.... I understand the premise but I still think that is literally the stupidest fucking thing I have ever seen. I dont even think that Zac had a thornmail. The Zac's argument was that,"The more health you have the more true damage you will take" then went on to say 8% of a 100 is less than 8% of a thousand..., yes that's true but vayne will do 8% regardless, so plz if someone gets the chance, Shoot me in the fucking head. :D