The effect of "Turbo Cannon" inflicts damage to the current controller of the monster when resolving the effect. If you chain an effect like "Enemy Controller" to "Turbo Warrior" to take control of the opponent's targeted monster, then the damage is inflicted to you.
Damage inflicted by the effect of "Turbo Cannon" is equal to the ATK of the monster while it is on the field. If an effect (like a Field Spell Card) is increasing/decreasing the monster's ATK, then the damage inflicted is half of the increased/decreased ATK.
The effect of "Turbo Cannon" which destroys the monster and the effect which inflicts damage occur simultaneously.