Or is he just being an SOB and doing it for fun, or because he can, or because he is mad at the world for losing his hands?
I think it was a combination of these three things. He might have had some ransom plan in mind, but it wasnt in place during the events of the movie.

With those super strong metal hands, jerking the gherkin would be a major worry for him.

The first Bond movies were the ones Mike Myers' parodied with Austin Powers, back in Connery and Moore's day the baddies sometimes did stuff simply for revenge or to show the world who was #1 villain. It's not like nowadays where Bond baddies always seem to have a plan to sabotage a rival so their own smartphones will sell better or they try to steal a warship to sell to the Yemenis.
The first Bond baddies were just good old fashioned nutters who had smelt their own farts for too long!
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He was angry b\c he couldn't pleasure himself (with those hands) or even enjoy the feel of a woman's soft body. That would suck.

If you can topple a rocket, you can also topple an ICBM. I assume he was testing before SPECTRE was going to hold the US and Russia for ransom. Get paid or else he will render their ICBM's useless.

Compensation for the loss of his hands.

He was part of a terrorist group that was trying to corner the world market on Nehru jackets and bring zee vold to itz neez!

Yeah, he was down and out but the U.S. government wouldn't lend him a hand. So he destroyed the missiles to try give them the finger, but that didn't work. Then Bond gave thumbs down to his recruiting speech at dinner. Then he realized, Crap! I made reactor overload-inducing switch too damn big! facepalm
(And this was all before hands-free cellphones, also frustrating.)

it was very clear in the movie that he wanted revenge from the american government because they refused to acknowledge his genius mind and refused to hire him.

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He is working for SPECTRE, the Special Executive for Crime, Terrorism, Revenge and Extortion. I suspect Blofeld, not mentioned in the movie, had some way of monetizing the whole operation but we are never told exactly how. Perhaps it was all just a big ball of wibbly wobbly crimey wimey stuff.