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.