Why is it Ventus? Really? What did he do wrong? And then Sora comes out of him. So... it's actually Sora? Sora is evil now?

 

HELP!?

From what I understand Ventus uses his armor to protect Sora while he is falling into darkness so instead the darkness consumes the armor instead which Riku defeats. Ventus is still asleep in C.O, it's just his armor.

A dream version, not Ven's actual armour. The armour symbolises protection from darkness and since Sora has some of Vens memories the armour comforts him and prevents him from entirely losing to the darkness. It turns into a dream eater because any concentrated darkness in the RoS turns into a dream eater.

ah. ok thanks people! That clears things up!

After fighting with Xemnas and falling to darkness, Ventus heart arrived and gave his armor for Sora to protection, but it seems that darkness took over armor.

No Sora's is not evil now but in order Sora not to go fully on the dark side ventus protected Sora giving him his armor. Now you fight Ven cause cause he may sees you like an enemy like sb that wants to harm Sora's heart and this is why you fight him. It's like our immune system when a bacteria is inside our body our immune system sends it's "soldier" to confort it so is Ven in this case he conforts Sora's heart from any harm.