Then you're up for felony charges on laws against boobytraps which fail to distinguish between lawful and unlawful intruders.
I have a big button on the front that says "Press this if you're a lawful intruder". It still sets off the thermite, but at least they have no one to blame but themselves, what else did they think the button would do!?
Yep. That totally solves everything. You are really fucking stupid. What sort of actual useful contribution do you have to make?
While I was being obviously facetious, if my data is incriminating enough to make melting down my computer worth protecting the data from prying eyes, then why doesn't thermite solve the problem? Many people are willing to die for their cause, so risking a felony charge to keep secret data out of the hands of the "enemy" is not unreasonable - and maybe a lesser charge than the evidence would have proven.
The thermite charge needn't injure anyone, your goal isn't to make a spectacular explosion, you just want to melt the parts that contain data - be judicious with the thermite and the triggering device, and maybe build a brick shell around the computer case with a chimney to vent the combustion gases outside. Put a sign on it that says "Warning, combustion device inside, do not move" -- if the police dismantle the safeguards and get injured, well, they should have read the sign first and taken reasonable precautions against injury. For the truly paranoid, make sure you destroy the RAM and every other device in the computer that could possibly store data -- including video cards, network cards, etc -- who knows what backdoors are there.
Of course, I don't *really* have a self-destruct system on my computer (I'd quickly get tired of replacing my computer and the cats that keep chewing through the GPS antenna wire and triggering the self destruct). There's no point in protecting my data against seizure by the government -- if the government wants the data on the computer, they already have it, seizing the computer is just a pretense so they don't have to reveal which back door(s) they used to get the data in the first place.