If “Hyper Hammerhead” attacks “Des Kangaroo” in Defense Position, their effects form a chain; “Hyper Hammerhead” is Step 1 because it is the turn player’s effect, and “Des Kangaroo” is Step 2. “Des Kangaroo” resolves first and destroys “Hyper Hammerhead”, then “Hyper Hammerhead” resolves and “Des Kangaroo” is returned to the owner’s hand.
Netrep Rulings:
A monster with lower ATK than the DEF of “Des Kangaroo” that attacks “Des Kangaroo” is destroyed as a result of the effect of “Des Kangaroo” if “Des Kangaroo” wouldn’t destroy it in battle normally.
A monster attacking with a lower ATK than “Des Kangaroo’s” DEF is destroyed regardless of the Battle Position of “Des Kangaroo.”