So, a though from Thanks To Them one thought keeps coming back to me.

It looks like Caleb used that archway portal to get to the demon realm while Phillip says he got there through a lake. (He is an unreliable narrator, but I have a feeling he wasn't lying about that.)

I have to wonder if Phillip had assumed that Caleb was kidnapped, not realizing he went willing with Evelyn. However, what if Caleb left that map and vial of Titan's Blood for Phillip to find so he could follow them. We don't know why he didn't just take Phillip with him, but maybe it was too dangerous because he was being hunted by the town witch hunters or something.

Maybe he left a note with a message that Phillip just never picked up the secret meaning.

Years later, Phillip gets to the Boiling Isles and we can assume took awhile to locate Caleb and he probably did not have an easy time of it.

When reunited, Caleb is delighted that Phillip finally made it, and can't wait to introduce his brother to their new family.

Phillip however is furious, assuming Caleb just out right abandoned him for this witch that seduced him, not realizing Caleb meant for Phillip to come with him all the long.

And in his rage either tries to attack Evelyn or maybe Caleb either way Phillip kills Caleb. Phillip argues he's justified because Caleb betrayed him first and blames the witches. He's not going to feel guilty about this, nope. No way. Not his fault, it's the witches fault and they will all pay.

If this is true and let's say that Luz and the gang find evidence of this, I'm willing to bet Phillip refuses to believe because then he has a choice to cling to revenge or live with the guilt the killed his brother over nothing.