A 10% pop drop isn't that bad really, especially if you are doing heavy blood hunting/patroling and have death scale. It does sound as though you may have had one of the 4% or 5% pop killer misfortune events as well. No biggie really...
I've had my capitol down to under 15k by turn 30 a few times, though most of it wasn't my doing exactly
If you tax and 200% and patrol to keep unrest down you will lose pop fast. In fact I'm surprised it was only 10% in 25 turns, though you probably didn't have those conditions for the full 25...
A better system to use is to designate a few 5-10k pop provinces as your blood hunting grounds and either patrol or just drop the taxes to zero (or something else low) to curb unrest. Usually wrecking your capitol in the early game is a really bad idea. Over taxing is also a bad long term strategy.
Every point of unrest killed by patrolling is something like 10 population. So with 200% taxes, you're probably getting 50 unrest or so a turn at least and putting it down, that's 500+ people per turn. Lose your population pretty fast like that - that's why it's usually preferred to lower taxes to decrease unrest if you have the time. Patrolling is for emergency unrest reduction and for catching enemy stealthers.