How was it justified for the Winx to not kill the Wizards in season 4?

Okay, I asked this swali earlier and only one person answered probably because I worded it really strangely, so I'm going to ask again just to see people's maoni on this.

In season 3 and in the first film, it was mentioned that the Ancestral Witches and Valtor did NOT completely destroy Domino (yes, it was a frozen wasteland, but still.) and the people were not dead; just imprisoned in the Obsidian Dimension. But, Bloom decides to destroy the Witches and kill Valtor because she believes that a (reversible) imprisonment spell and her realm being turned into a (reversible) wasteland was a good reason to do so.

In season 4, it was mentioned that the Wizards of the Black mduara, duara executed countless fairies, cut off most of their wings, and imprisoned them for centuries on Tir Nan Og. Yet, the Winx decide to help the Wizards and turn against the warrior vichimbakazi because they state that "vengeance is not the way" even though Bloom herself got PLENTY of vengeance in season 3. They even take the Wizards' side.

So, basically I don't get this. Why would the Winx, who have killed and imprisoned plenty of bad guys in the past, suddenly think it is justified to side with the bad guys and turn against the people that they murdered and tortured? I know it's just a cartoon and all but this REALLY doesn't make sense to me.