Why didn't Itachi spare Fugaku and Mikoto?

[Torche]

no it wouldnt. nonsense.
itachi didnt have to do anything.
either way konoha is weakened, and a foreign nation will invade eventually

If Itachi didn't, then Sasuke would have been exterminated with the clan. He killed the clan, so Sasuke could live.

 

[Gyakusetsu]

he killed his parents to turn his brother into super villain?

The entity controling Itachi's actions. That entity also needed Naruto to experience the same thing hence the Kyuubi attack that claimed his parents and forced him to early life isolation. The parallel is not coincidence.

 

[AGoodBoy]

True, but with their clan dead, they couldn't harm the village and complete the coup, right?

They were leading the coup........... If they formed one coup, theycan't form another....? Even if the uchiha clan was gone, they could then them as matyrs to rally another clan(s) "See what the village does to those who oppose it? Our clan was seen as a threat and we were promptly exterminated! Only our family survived thanks to the skill of our most proud son, Itachi, however he went missing after the event... We feat the elders succeeded in taking away a piece of our family. We need to put an end to this; We are konoha citizens... *blah blah blah more speechy shit*". Then it's Good bye konoha.

Itachi's parents were one of the most important people that needed to die. You don't go killing all the pawns and leave the king standing. That's just a bad idea.

 

[USSJ Future Trunks]

If Itachi didn't, then Sasuke would have been exterminated with the clan. He killed the clan, so Sasuke could live.

and then he pointlessly tortured sasuke to madness and made him a bigger threat than his clan.

 

[Torche]

and then he pointlessly tortured sasuke to madness and made him a bigger threat than his clan.

No, he tried to unlock Sasuke's MS to further him, but that obviously failed.

 

[USSJ Future Trunks]

as i said, he was trying to make sasuke stronger at the cost of his sanity.

 

[Torche]

The Uchiha get stronger with hate, Itachi tried to use this to his advantage by making Sasuke focus that on him. He would use the "curse of hatred" to his advantage instead of the hate controlling him. Sasuke never went insane after the Tsukuyomi either.

 

[TRE MERCER]

I guess brotherly bond>That of a parent in child which is bullshit.