With Taunt and Bloodthirsty, first you check to see if the action is possible. If you can attack, then you must do so, instead of choosing any other action.
If the attack is not possible, you are not required to do anything which makes the attack possible. For example, if the attack requires you to pay mana, and you could destroy a decoy to gain the 2 mana you need, you do not have to do that.