is kayn supposed to be able to melt ppl in before they can click an ability?

Just had the weirdest game in LoL imo ever. I have been away for a year so some things are different to me so correct me if I just don't get something. But Kayn could melt anyone on my team before I could press my R as Sona in many skirmishes (except maybe the top laner, but still quite fast). What was odd about this is that he got a few kills and was able to do this but wasn't actually fed yet. Infact, my team was doing quite well. Jinx got 3 kills early on and one of them was even Kayn, and yet that didn't deter him. We were pushing into their base long before they were ours. And yet, it didn't matter. Tbh it felt like I was summoner level 1 again, except my team wasn't playing bad and feeding.

Once I did get him in my ultimate, he was stunned and 3 champions, including Jinx with kills, was wailing on him. Except he just disappeared, came back and all three champions were dead.

Pantheon managed to kill him a few times and his teammates. But then in the next fight he just rush the less tanky people and melt them down.

I really don't know what to make of this. I was thinking about getting back into LoL, but is the game really this imbalanced now? Or was this guy just so super good a player that he could pull of this level of play against a team not feeding, not playing bad and actually winning? Yea, we were kinda winning until that one player went on a rampage.