I cannot understand the sory in ToB. If you are Evil Character than you could join ToB Final Boss&CO. to ressurect Bhaal.
So where is the roleplaying went?

Basically, the story in ToB is just about closing the saga about the Bhaalspawn and getting rid of the Five. For a more-indepth story, read more but it may spoil some.
Spoilers...
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In the Throne of Bhaal, the story itself is centered around you, you are the one Bhaalspawn alone to lead your group to stop from the prophecy from becoming true of Bhaal's ressurection. If you were killed, the Five would slaughter all the rest of the Bhaalspawn and enough essence would collect where Bhaal would live again and make his Five strongest children Demi-Gods. That is what is supposed to happen in theory, but Mellisan being the high priestess wouldn't allow it to happen because she is the sole person that knows the incantations that will form the essence to ressurect Bhaal and wants the power for herself. Regardless of your alignment, good, evil, chaotic-nuetral, this is your role in the game until the end. But depending on your alignment, you can take the essence for yourself in the end and become a god of good or evil.

If was an evil character, I wouldn't want to resurrect my dead father and be ruled by him - I'd want to be a god myself. Perhaps that's why the PC doesn't resurrect him.

You are probably right

In order to "role-play" an Evil character in ToB, you just have to be a selfish Evil character. You have to think along the lines of "I'm not gonna share the power with The Five... I'm gonna take it for myself!".

Granted, it may be hard to role-play a Lawful Evil character (who might want to join The Five & help them with their goal), but a Neutral Evil ("killing the others might grant me more power!") or a Chaotic Evil ("heh... I'd kill them anyway!") character should be pretty easy.

The truly impossible alignment to role-play is True Neutral. There are no "maintain the balance" options.

- Imrahil

Well said, nuetral characters may seek to maintain the "balance" between good and evil but the truth is they're only on the side of good. You'll never see a nuetral character killing off good fighters or wizards so evil can have an equal chance to win in a fight.