I sort of agree but as Mathias said, it would be too unfair. A lot of people don't know you can fish for xp and therefore this is a hidden aspect of the game (which is good). When you fish for xp it can take you sooo long to get enough for one level and you're usually rewarded with sharpness 2 (Unless you have TWO levels and enchant TWO diamond swords and get SMITE on BOTH of them! FUUUUU). Personally I don't like this idea as it stands unless you get as little xp from killing as you do fishing.
EDIT: Just remembered, when you die, Minecraft has rule that makes it so you only drop like 20% (give or take)of your xp so killing people with lots of xp won't give you that much.
EDIT EDIT: I meant to say Sharpness 2...