They don't retain their human personality as necromantic magic is animating them.
p315 of the monster manual has a more detailed description of zombies.
Dark Servants. Sinister necromantic magic infuses the remains of the dead, causing them to rise as zombies that do their creator’s bidding without fear or hesitation. They move with a jerky, uneven gait, clad in the moldering apparel they wore when put to rest, and carrying the stench of decay...
A zombie retains no vestiges of its former self, its mind devoid of thought and imagination. A zombie left without orders simply stands in place and rots unless something comes along that it can kill. The magic animating a zombie imbues it with evil, so left without purpose, it attacks any living creature it encounters.
Since their personality comes from the magic animating a zombie not from the soul (which is off in the afterlife or somewhere else) then if you increase their intellect, they'll still want to attack living creatures. They may have the intellect to do so in a more nuanced fashion, using lies, ambushes, and complex tactics to achieve their desires.
Of course, there's a lot of flexibility for DMs to handle things in a different way. Their undead might retain vestiges of personality of the original person, to add some extra horror, or they may decide that the zombie's hate of humanity is removed when they are polymorphed, as they are now another creature type and are no longer animated by necromantic magic.