Should a tank one shot people?

First time i post somenthing like this. To put a bit of context i'm not an adc player, but i do play the role here and there in normals or with friends when we all go fill. The matter of the question here is that i've encounter a couple (to say it nicely) of Shyvanas going full tank and getting a Guinso's, and literally one shoting me or the midlaner. Healing like a low health WW, damaging like a fed Twitch and tanky like a late game Nautilus. Anyone find this in their games too? Does it happen with any other tank? And now, im not talking about them dealing damage or healing too much. No no, i'm talking about literal one shots, like the old crit Rengar. Or early game Renekton maybe.