Even though it roars and carries on a lot, the yew tree monster means well. It's like the monster with a heart of gold. It comes when it's summoned by those who need it—at the monster's discretion, of course. As it tells Conor, it doesn't do this for just anyone. It knows Conor truly needs its help.
And although the monster has a tendency to tell convoluted tales whose meanings aren't immediately clear, they do relate to Conor's life. It's just that it sometimes takes Conor a little time to figure out how. It's worth noting also that when Conor's in the room with his mother on her deathbed, the monster comes back to be with him, to remind him to tell her the truth. We think the monster's done and gone, but it still cares about how things turn out for our Conor. We could all use a guide like that in our lives.