Many people argue is Seifer act on his decesion or Ultimecia possessed/play trick on his mind?The truth is Ultimecia never did that.Seifer act on his decision.If Ultimecia can do that why she dont do that to Squall and Co(i.e when they in D-DIstrict Prison).What Seifer did is not wrong or right.In war there`s no good or bad side just two sides holding different views.He maybe realised that what he did is wrong but he is Sorceress Knight after all,he must protect his sorceress(one of the code of Sorceress Knight).But eventually he realised that he is only a puppet,a helpless puppet.It looks like her sorceress betrayed him.
PS:I have read a wonderfull fanfiction about Seifer by Darren "Edge" Shier here it is:

And that is it, really. From Edea's point of view. To this day, nobody has a clue what went on
in Merris's home that fateful night. Merris had not been depressed about the child. She was just
confused and distressed. There were'nt that many virile men in Alanga. And she had slept with
none of them. Was this a virgin birth? Or could she simply not remember? She should remember
something like that. In any case, the birth passed with a surprising minimum of pain. The midwife
who was named Legatia, was very superstitous. She had lit a fire, but the room became very cold
after the child had been born. Suddenly, the fire went out.
"What is this?" asked Merris.
But Legatia begen to chant in the old language.
"Liberati Mei." she said, feeling a presence fill into the room. "Liberati Mei Infernus."
"Words." a terrifying voice came from nowhere. "Words are just fantasy. This is reality. Words
kannot help you."
At this, a symbol of fire ignited in the floor. Two equilateral triangles that
overlapped, sort of similar to the Star Of David, but this symbol had a goats head inside it. The
kind that that satanists often used. The fire began to burn higher and higher, hotter and hotter,
beginning to take on a human shape, until there was someone standing there, as the flames died
down. She was a Sorceress. Dressed in red, with a pair of dark wings that trailed behind her. The
wings looked like those of a crow, not an Angel. She had long silver hair that came all the way
down to the ground. She was truely the most terrifying sight that Merris or Legatia had ever
seen. Her eyes blazed with the colour of gold, and her eyes had all the seeming of a demon's that
is dreaming. She looked at the humans with utter contempt.
Contempt for humanity.
Contempt for life.
"W-who are you?" asked Merris.
The Sorceress smiled evilly with fangs of diamond. "The wind that howls in the night." she said.
"The monster under your bed that makkes you wet your sheets. Some kall me a witch. Some kall me
a demon. Some say i have made a pact with the devil. Some say i AM the devil. Thay are all
right. My name is Ultimecia."
"What do you want of us?" pleaded Merris.
"My Soothsayers have given me a prophecy." announced Ultimecia. "They klaim that the one who will
destroy me was born in this time. The tell me of dreams. Dreams of a lion. Sure enough, he is
being conceived as I speak. So...i need a knight. A rival for the lion. One who is strong enough
to face him. Strong enough to defeat him."
Merris realised Ultimecia was talking about her baby. "S-Seifer?".
"I won't lie." said Ultimecia "I played a part in his kreation. Bring him here."
The women watched in horror as Seifer levitated off the bed and floated into Ultimecia's arms.
She cackled like a witch, caressing his head with her fingers. Then they realised that she did'nt
have fingers. They were talons, twice as long as a human's fingers and razor-sharp.
Legatia advanced on her, brandishing the symbol of Holy. Ultimecia turned her head and stared at
Legatia, who froze on the spot. Ultimecia did'nt seem to be looking at her. She was looking INTO
her, looking right into her soul. The symbol of Holy crumbled to dust under the might of the
Sorceress's gaze. Her eyes were not just yellow anymore, they were glowing, like halogen lamps,
burning bright, so bright that Merris, who had turned her head away, screamed at the blinding
light. Ultimecia continued to pierce Legatia's soul with her eyes.
"Die." commanded Ultimecia simply. It was not a threat, not a request, not an order. And Legatia
fell to the floor, dead before she hit the ground. "Seifer is going to be my knight." cackled
Ultimecia. "Are'nt you, Seifer? I'm going to make sure you grow up big and strong. The mightiest
soldier there ever was. With my silent guidance, you will be the greatest."
"Seifer will never be a soldier!" defied Merris. "I'm going to raise him as a poet, or a
farmer."
"I am afraid that your part in Seifer's life is over." announced Ultimecia.
She held Seifer in hers arms as Merris ran right at her. Holding Seifer in one hand,
Ultimecia grabbed Merris by the throat with the other. She lifted Merris up into the air.
"You worthless creature" she hissed. "I give you, a pathetic human, the honor of
birthing my knight, and you have the gall to think that you can take him away from me? ME?!!?
He would not even exist without me!!!!!"
Her eyes began to burn again. "Your life is over." she commanded. "You serve no more purpose in
Seifer's life. Go into the bathroom, lock the door, and drown yourself."
Merris complied, without a nerve of resistance. As the door locked and came the sound of running
water, Ultimecia cackled, cradling Seifer, who gurgled innocently.
"Innocent, i see." growled Ultimecia. "That can be remedied. I will see to it that you will love
the sound of battle. You will relish the smell of your enemies blood. You will not acknowledge
other mortals as your equals. And Death will be your travelling companion, following you wherever
you go.
The one of this time. Edea. She has a great many part to play in my plans yet. She will
take you back there. To that place. The place where the lion will be. You and him will grow as
parallells, but you will be stronger. And you will be given his abilities. I shall see to it
that you will have all the training, abilities and knowledge of a SeeD. But, by the Demons,
i will never allow you to join the disease that is SeeD. Never.
You will stand by my throne one day and i will watch as you defeat the lion. It is your destiny."

The baby gurgled with glee. Indeed, the future was bright for Seifer Almasy....