Am I overthinking or is he in the wrong?

I’ve been with my husband for 11 years. During our first 3 years of relationship, he had a girl best friend that constantly caused problems in our relationship. She claimed he wanted to cancel plans with me anytime she wanted time with him, falsified rumors about me to make him want to split, and contacted me only to ask if we were still together. I ended up getting pregnant after the first year of dating and during that pregnancy , she was ruthless. She told me she hopes my baby dies and when we would make plans together she would show up uninvited. The last straw, I asked my husband (boyfriend at the time) to set boundaries or end the friendship. He ended it after finding out she slept with his step-brother. About a year later, he contacts her behind my back and apologizes to her and blames me for the friendship ending. We fight it, we break up, and he has her find a friend he can sleep with within 3 days of our breakup. We end up forgiving him and in the process he ends his friendship again with her claiming hate towards her. We no longer contact her and years go by until yesterday. My husband saw her on Snapchat and checked her out claiming it was only to see how she’s being doing all these years. Of course I was extremely angry at him and we got into a huge argument over it. To me it’s disrespectful after everything that she put me through in this relationship and what he allowed it made my first pregnancy and first years with him living hell. I forgot to add that she also destroyed his previous relationship with falsified claims and he had a huge crush on her (she never wanted to date him only lead on) He claims it’s normal for people to want to check on how others are doing in life. I could understand people from the past but specifically her? I’m convinced he’s still secretly obsessed with her but stays with me because I’m not promiscuous and she’s married now. What should I do? Am I wrong for overreacting and getting upset over it? Is this normal?

Am I overthinking or is he in the wrong?
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