I am not feeling sorry for him, in fact, I like seifer when he was in the ending scenes.

The whole Seifer that we see till disc 3 is the "Childish" seifer. Remember when Edea said that part of him wanted to stop and part of him want to continue? Even Edea said "Bit farewell to your childhood", I believe it meant that she pulled out his childish ambition and dreams. Like when we small, we play Cowboy and Indian and we thought that killing Indians is a good things though it was wrong. So instead of being mature, Seifer was pursuing his childish ambition to be sorcerer knight and killing all innocent people.

In the end of the game however, we see the mature Seifer. Seifer who realised that he has done wrong and still being forgiven by his friends in the garden. That is why he smiled when the garden passed.