I don't think this really fixes the problem. The monk still knows how many ki points she has, so can be careful not to kill anything to avoid going over her limit. Especially since any time you get a "killing blow" with a melee weapon, you can choose to simply knock the creature unconscious.
Even if you make the Dragon tattoo unable to simply knock a creature out (which works well thematically), you can generally figure out the maximum number of things you're going to kill with each attack, and it's almost always 1. (Barring the sack of rats/flaming anthill scenario.) So after your ki recharges for the day, you spend one or two ki points flurrying against a tree or whatever, and the drawback almost never kicks in.
If you're fine with that, then that's great. Personally, I'd rather it have a drawback that has a decent chance of kicking in, even if it had a much lower cost. If I were going to use this, I'd probably give it something like this:
"On killing a living creature with a melee attack, the wearer of the Dragon tattoo may devour that being's Ki. If the wearer chooses to do so, he recovers d20 Ki. If the amount rolled would put the wearer above his maximum Ki, his Ki becomes its maximum and he takes d10 force damage for each additional point of Ki as the excess energy tears its way out of his body."
This doesn't capture the idea of an ongoing "Ki imbalance" the way yours does -- but realistically, 10d10 damage per round isn't going to last very long before it kills the character anyway, so it's not going to be "ongoing" for very long anyway.