Jannika,
I'm not sure what you may have done that was deadly but drip acclimation done properly will actually be reducing/diluting any ammonia level because you will be constanly adding/removing some of the old, then mixed with new water with each little PWC.
Lets consider this example.
You bring home a fish and have a pint of water with the fish. You put this pint of water and fish into a larger container and start the drip.. or even adding an ounce or two at a time from the home tank.
If the pH of the fish store water was 7.0 and your tank is 8.0, when you add one ounce (a 7% increase) of water from the tank to the fish store water, that would only change the pH by a 7% between the difference from the fish store water to the tank water... so it should only raise the pH from 7.0 to 7.07. Then when you add another ounce, it raises it to 7.14 and eventually up to nearly 7.5 BUT you will also be removing some of the old/new mixed water as you add in more new water so you will constanly be dilluting any ammonia in the fish store water. Also, I add a small decoration or plant to my acclimation container as that will have nitrifying bacteria growing on it... or the plant will even use up ammonia. Eventually, after enough of these little bitty PWC's, the fishes water will be 99% the same as the tank's water and then you can feel more comfortable knowing that shock will not be an issue. Over the course of an hour or two, depending on the starting differences, it gives the fish time to acclimate to the drastically different water parameters.
I have moderately hard water with a pH in the upper 7's to 8.0 most of the year so ammonia can also be a problem for me when acclimating new fish.. although not as much as someone with a pH in the low to mid-8's... but I've never lost a fish during a slow acclimation process. I've seen LOTS of online threads where fish died and were likely shocked by either pH or osmoregulatory shock when not acclimated properly. pH shock is more common when going from high pH to low pH but osmotic shock works both ways from all I've read.
He already lost some fish when doing a quick acclimation.