Worst first post ever.
Casascius has been around since just about the dawn of Bitcoin. AFAIK, there have been exactly 0 accusations of him keeping and using coin privkeys (keeping in mind Goat, one of the most temperamental Bitcoin users of significant public awareness, has had thousands of bitcoins in Cas coins).
I like Casascius and everything but I don't really agree with this assessment. He could have kept all the keys, waiting for the coins to be valuable enough to steal everything. Right now there are tens of millions of dollars worth of his coins; few people have the integrity to walk away from such amounts, and past performance (in this case, Casascius' good form so far) is not an indication of future results. If he kept the keys, he has much more to gain by stealing everything than to lose.
Of course, this threat model is still much better than if he could make the decision to keep keys retroactively.
I don't disagree with the principles of what you're saying. It's just... I don't see Mike running off with $100M in stolen bitcoins (but as you know, I've been wrong before, though I've never had the level of confidence like with Cas). I'm pretty sure he's rational. It's not something where he can just say "oh, it was uhhh... a relic hunter who was spying on the manufacture of the coins." He's a criminal from the moment someone discovers coins are missing, and the amount he'd have access to would almost definitely draw international cooperation from law enforcement. Beyond that, he's been a long-time supporter of Bitcoin, and I'd be very surprised if he didn't have a good few heavily-loaded, legitimately-obtained coins for himself. I guess I don't know, though. Maybe he does a hundred lines of cocaine a day and is really motivated to get coins by whatever means necessary. (can't wait to see his response to this thread)
(in looking up info for response, it turns out Mike suspended operations a couple days ago.
https://www.casascius.com/ )