Remove bestow target, does it become a creature?
It becomes a creature, because the rules make a special exception for bestow. Even though you've cast your creature with bestow as an aura spell, when it resolves, it can end up as an aura or a creature. From the bestow section of The Mechanics of Theros:


  If the target creature leaves the battlefield after you cast a card with bestow as an Aura but before the spell resolves, the Aura spell will resolve as an enchantment creature rather than being countered like a normal Aura spell. If the target creature is still on the battlefield when the Aura spell resolves, it resolves as an Aura enchanting that creature.


And the specific rules for this case:


  702.102a ... These effects [turning it into an Aura spell] last until one of two things happens: this spell has an illegal target as it 
  resolves or the permanent this spell becomes, becomes unattached. ...
  
  702.102d As an Aura spell with bestow begins resolving, if its target is illegal, the effect making it an Aura spell ends. It continues resolving as a creature spell and will be put onto the battlefield under the control of the spell’s controller. This is an exception to rule 608.3a.


(Rule 608.3a is the rule about how aura spells normally resolve, which would cause a normal aura spell to go to your graveyard at this point.