why is mat angry with maria (end of ch 12)

When they hear mewling in the distance, Maria believes they are experimenting on cats and insists on rescuing the animals. Tom opens the closed door from where the sound seems to be coming and reveals a boy strapped to the bed making the noises. Tom says it is MacGregor's clone, not a boy, and he comforts Maria, leading her away and leaving Matt with the clone. Matt follows and tries to comfort Maria, but she refuses to let him touch her. Tom leads Maria away triumphantly while Matt listens to the clone in dejection before following them.