Caught someone stealing self checkout.feeling partly responsible ?

Here's the story. I was working at the self checkout . during my shift there are two people with a trolley full of items . and you know how the error messages with the weights tend to come up. I quickly look over the bags and fix the issue. The error happens like constantly and my supervisor told me to keep an eye on them. Honestly I was so frustrated because of the constant error. Turns out on one occasion the person I don't know whether by accident or not put in the extra item in the bag. In the end i offered to scan the rest of the items because well I'm faster and there's a long queue and I still have to assist others as well. I left them to themselves up to where they have to make a payment as I was assisting other customers. Shortly after my supervisor asked me to print out an extra receipt. He scanned all the bags again and turns out they were in fact stealing quite an amount.

My question is. Am I responsible for this to happen ? It was my job to supervise that area and I failed at it. I feel partly guilty that I couldn't avoid them from stealing.

Any advice ? And I'm feeling like ****. And I still have to return back to work