It doesn't require a criminally minded 3rd party coming onto someone's "safe" property to do something horrible with a sawzall and/or oxy-acetylene cutting torch.
Plus, they’re ubiquitous, you don’t know who has one, max damage is minimal even worst case — go fish!
We live in a world where multiple people are killed every year by tipping vending machines over onto themselves and you propose to make nuclear reactors a mass market consumer good that goes in everyone's garage?
But I can’t disagree that it’s more exciting to imagine terror dreams.
A big potential concern with small reactors are highly toxic radionuclides; those can be much more dangerous (with LD50 far under 1mg/kg) than "ordinary toxins" like bleach or even nasty stuff like methyl isocyanate. That means expensive disposal and protection measures.
All of this is a non-concern though because there is no realistic path for nuclear reactors to compete with PV+batteries, ever. With cells already <$100/kWh and the panels being cheaper than glass windows, we will never be able to build, maintain and dispose of nuclear based reactors tech at a competitive price point, especially not with the insane current battery demand (automotive) driving technical optimisation and price competition.
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.