ma, I sorry

When my son has an accident I want to be mad. Not violent angry, just mad so he knows its bad. As I clean up the mess he sits there in shame and says, "ma I sorry" as clear as day. He keeps repeating it until I acknowledge his sorrow. How can be mad at a child with his head down and shame written on his face, that says, "Ma I sorry"? Today I was washing undies in the toilet (Sorry if you don't have a toddler and find that utterly disguisiting, if it makes you feel better your mom probably did it when you were a toddler.) He thought it was funny, I said very sternly, this is not funny, it is bad to have an accident. He immediately stopped laughing never to start again, saying, "I sorry" "Ma" Ma?" (are you listening?) I responded yes, then he said once again "I sorry." He seems genuinely sorry, so how can I be upset with him? It makes me want to wash my hands and hug him, instead of being annoying how gross it all is.