He claimed it. And he is the one capable. And he is the one with the motive, since he hates shitcoins (but he doesn't waste his time destroying them unless they proclaim to challenge Bitcoin).
Eh, you don't need a strong motive, do you ? If I would have been capable, I would have done that too, just for the fun of it. Crypto is there to be challenged and to be played with, right ? If I would know how to screw up the bitcoin block chain, I wouldn't hesitate to do so. Crypto being a system that has to function in a trustless and hostile environment, there's nothing even morally wrong with challenging it. I would assume that there are many, many people out there with the same sense of making fun as I do, and who are way way more capable, so I wouldn't look for a motive. It is normal. It is a fun game.
Of the people with the balls to stand up to the potential legal risks and the resources to do it well, he is one of the few with the motive, given he promised to destroy any fork of Bitcoin which didn't agree with his principles. Ethereum was touting to challenge Bitcoin. He put his foot down. Vitalik was squished under his big toe.