The worst thing an evildoer can do is blow up your own house and the few around it. and no one does that because you go to jail forever.
The worst thing you can do is let it melt down, which means it quietly shuts itself down.
No, they can take the isotopes out and dump it into your local water supply. Or if they're suicidal and the isotopes have been encapsulated in some sort of tamperproof system, grid the whole thing down to granulated powder using less than $20,000 of power tools (disregarding their own health and the entire nearby area, of course) and then dump it into the local water supply.