Why did Heathcliff and Catherine argue?

In Chapter Eleven, when Heathcliff visited Thrushcross Grange, Nelly saw him kiss Isabella in the courtyard. She told Catherine what had happened, and when Heathcliff came in the two had an argument. Heathcliff said he had a right to do as he pleased, since Catherine was married to someone else. He said: "You are welcome to torture me to death for your amusement, only, allow me to amuse myself a little in the same style" (112