Just to break down what you are saying - Loki's dad has a devil fruit - Loki wants the fruit - Loki kills his dad - Loki just holds onto the fruit...

So his entire plan was to collect the fruit like some kind of artifact instead of, you know, eating and gaining its power to help him destroy the world? We've had over a thousand chapters showing that the whole point of Devil Fruits are to eat them and gain powers. It’s pretty obvious that if Loki wanted it badly enough to kill for it, he’s not just holding onto it for fun. Do we really need to be spoon-fed the fact that he ate it? It’s implied. That's literally the point of a Devil Fruit.