Instant death triggers and instant creature control aren't present in on-element Death cards for a good reason. Many Death cards benefit from the death trigger, and their effects are stackable. Sure, it's fine for the early game, but once you hit mid-game and can activate multiple cards' abilities with a single death trigger, it easily snowballs out of control. This is also why Aether's creatures are either immaterial, fragile, or hard to cast (thanks to creature-enablers like Fractal and Parallel Universe).