Because he's angry, he's taking Steph's death out on everyone and he blames Bart for making Dom do it, even though tecniqually no one made Dom do it (Bart shouldn't have bullied him) but still, when some people grieve they take it in different ways.
Lol I quite like Lee. Can't stand leanne, jem, "Jason", mitzeee, Texas, gabby and her kids, that new headteacher and his kids... The list goes on