The conflict is that Patrick was going to leave Mary while she was 6 months pregnant. But the main conflict is that Mary kills her husband by hitting him with a frozen leg of lamb. It is an example of external conflict.
There are many themes for this story... there is betrayal from the husband trying to leave his wife. There is Justice vs Injustice they both suffered injustices like he tried to leave while she was pregnant with their kind and his injustice was being killed by his angry wife. One other theme is that first impressions can be misleading and Mary is definitely a good example of that.
