Opinions please!

My daughter will be 5 in April and today her two cousins came over. One of her cousins is 1 and today my daughter decided for the second time to lock the 1 year old in her room.. the first time it happened she got in trouble of course but it didn't seem to do anything because she did it a month later. I do discipline by spanking and this time I spanked her a few times for this because I felt like it was a serious situation. I feel like a terrible mom but I feel like she's old enough to know better and she seems to find it funny when she does this until she gets in trouble than she's upset. Do you think I overreacted or do you think I was right with my punishment? Also I should add I don't spank very hard and it never even puts a red mark on her butt.