Is it normal i can't stop thinking about this?

I live with my roommate. She owns three dogs. Or, owned. One dog, Korki, constantly tried to get out of the gate. But whenever he did, she called him and he would come running back to the house. About a week ago, she wasn't home, and I opened the gate to let my sister in to use the restroom. We were going to my other sisters rehearsal dinner that day, she would be getting married two days later. He escaped, even though I tried to block the opening with my leg. He started running as fast as he could, as was I. I was calling him over and over and over again, but he just kept running. She wasn't home to call him back, and he doesn't listen to me whatsoever. Eventually he reaches the main street and runs out in front of a car that wouldn't stop, despite me screaming and yelling. I saw him get run over, and I immediately start crying. The lady in the car eventually stops, gets out and starts to yell at me, asking why I didn't keep him on a leash. She eventually speeds off, saying she's late for work. I called my roommate, and tell her everything that happened. She starts yelling at me as well, telling me I killed her dog. At this point I'm sobbing, apologizing to her while she continues to cry as well, telling me I don't know how much Korki meant to her, that he was like her son. This happened a little under a week ago, and I can't get this out of my mind. I feel terrible about the whole situation, like it's all my fault. Korki was a very mischievous dog, he always tried to get out, and I tried to explain that to her, but she kept telling me it's all my fault. I've lost my pets before, and I know how it feels. I'm so sorry that this happened. I can't tell you how many times I've apologized to her. I bought her flowers and a card that night. She still rarely talks to me any more. She's had him for two months, she got him after her mom died. I feel like such a horrible person.