I just had a game with Canadian byo-yomi. I almost never use it, but this time my opponent insisted on it.

We had ten minutes to play 25 stones, which is easily enough for me. Not so my opponent who had at least 10 stones to play in the last minute. This meant they would go for blitz-style play half the time, including what became ever more apparent as timesuji.

The fourth time, there were 15 stones to be played in the last minute. I played the most complicated thing I could think of (about ten moves deep, it doesn't actually work, but if it did, a too large group would die) and the time constraints let me win.

The case for bad time management is clearly there (the last 5 moves would have to be played in 4 seconds... manual dexterity became a factor!) but this might as well be a case of trick play. Even if my opponent would have been able to play fast enough to reach the next period, they made mistakes under extreme time pressure, thus giving me an advantage either way.

So, my question is, is it my opponent being a fool for not managing time better? Did I do something unethical, deliberately going for the time win?

Or are both parties guilty, of their own separate follies?

You opponent insisted on those time controls and maybe loses that way a lot. I don't see an issue with what you did.

You did just fine. Both players agree to the time constraints before the game starts. Some people use a large junk of Candian time to evaluate the board without being under time constraints per move while doing so. They then know how they want things to go and play their moves quickly. This does not seem to be the case with your opponent, but that is what he chose.

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Still officially AGA 5d but I play so irregularly these days that I am probably only 3d or 4d over the board (but hopefully still 5d in terms of knowledge, theory and the ability to contribute).