Ezreal's state of balance is due to a bug.

A lot of people haven't gotten the memo; Ezreal is not breaking Klepto. Muramana is breaking Ezreal.

When Muramana is charged, every single target spell and attack consumes 3% of your current mana to deal double that in physical damage. 3% of Ezreal's mana pool around the time he's fully stacked (let's say that's around level 13, though that's probably a bit high) is about 1800. This means a full mana Q will consume 54 mana and deal 108 damage. For that early in the game an extra 90-110 damage on your Q, auto, and E totals to just shy of 300 extra damage on a quick combo so that's really respectable damage.

But currently he's bugged to double dip into Muramana, for double damage (not sure if it consumes double mana but that's not too important). Essentially, on a full mana EQ auto you're getting almost 600 extra damage. Single Q's are proccing for over 200 extra damage and when you're mid-game on any squishy and you have 1200-ish health, you're getting hit for ~200 from Triforce, 200 from Muramana, and another 200-ish from Q itself. It fucking hurts. Klepto is good on Ezreal, but it's only abusive because he's getting a bugged item that much faster.

My advice is still permaban him though because for some reason Riot decided not to patch or disable Ezreal.