Why are people like this?

My mother in law told me that when my wife and her 2 sisters were kids, she used to give them her cough syrup at night to help them sleep. the next time she went to the doctor, she said she needed a refill and the doctor was like, that bottle should last you a long time, are you taking it every day? she was like, no, i give it to my kids to help them sleep somtimes. the Dr was like, DO NOT DO THAT, this cough medicine has codine in it. Needless to say, she stopped.
But this dude KNOWINGLY put sleeping medication in their drinks? If i was a parent i would be pretty ticked if someone knowingly drugged my daughter. If i had to guess he probably did this to his daughter all the time.