Big mistake by mil

my mil recently looked after my 22 month old whilst I popped to collect some photographs. i was only gone for around 45 minutes. On the way home I noticed her car parked in the car park of the local Tesco express so I pulled in. I found my baby sitting alone in the back of an unlocked car, mil obviously in the shop, couldn't see him. He was eating a bag of quavers (could have choked) and also I noticed his car seat was not securely fastened in a d his straps were very loose. I then proceeded to "kidnap" my son and took him back after about 5 minutes, to find her and several other shoppers frantically searching for him. The point is I'm now being told I'm sick and evil by my sister in law and that what I DID was "bang out of order" just wondering what you all think and how would you have reacted?