But I gotta throw in a vote for his mother too. I think his mother getting killed was a pretty important reason. It was his freaking mother.
This.
Frankly, I just hate how everyone makes a big deal out of Aerith's death being Cloud's main motivator for killing Sephiroth. It was tragic, yes. It was sad, yes. But Aerith wasn't the only thing Cloud ever knew in the world. Aerith wasn't the only thing Sephiroth destroyed.
As a super-AZN, I have to say that family ties are really important to me. Japan has this mindset as well. I know that Cloud's mother didn't get enough mention as she's supposed to. Heck, they never even mentioned her beyond the flashbacks. I kind of didn't mind it, as most protagonists never even have parents (they're mostly orphans). But I think deep inside, Cloud still has his mother in his heart.
In a way, I think Cloud projected his mother onto Aerith. She's described as a mother figure to him and the planet, and he even calls her 'mother', right? By witnessing her death, and failing to protect her, he sees his own failure to save his mother in the situation, and so feels that way.
I say Aerith is the trigger. Like, the final straw. The weight of everything Cloud lost came crashing down on him at the moment of her death. But Aerith isn't the NUMBER ONE SUPER DUPER MAIN REASON THAT CLOUD WANTS TO KILL SEPHIROTH. Avenging her was part of the plan. Save the planet, get revenge for everything that's been lost, avenge Aerith--this comes with killing Sephiroth once and for all.
Let's not forget that AVALANCHE as well fought for the planet and Aerith's death. It wasn't just Cloud. We can even say that her death became
their main motivator. I mean, most of their troubles came from Shinra. They have no reason to kill Sephiroth other than to save the planet. Even then, they were just tagging along Cloud chasing him even before Aerith died.
Just a rant.
Aerith isn't everything to Cloud, gaiz. Cloud had other losses, other pains. He has a whole list of legit reasons to kill Sephiroth. Aerith's death was the trigger, not the main motivator much less the only.
I love Aerith, but let's not put her on a pedestal. Just saying.