In Avengers: Infinity War Red Skull tells Thanos that he must kill someone he loves in order to obtain the soul stone.
But if you truly loved the person you would never kill them. Can someone explain this paradox?
if you truly loved the person you would never kill them
This is, I think, overly simplistic and I think you missed the point of that scene.
Thanos killed Gamora despite loving her....because, in his mind, it was for the greater good of the universe.
That's why it was called a sacrifice.
Gamora was convinced that Thanos couldn't love anyone and this would be the fatal flaw in his quest.
Then she realises, in the same instant, that she's both wrong (and he does love her..and is quite possibly the only thing he loves) and he's going to kill her anyway!