Halo is about a person who is very self righteous. It is very likely that this person is someone who is involved in a religious war, such as the "Muslim Conquests" or the "Jewish-Roman Wars".
The song is sung from the Self-righteous persons point of view.
For References sake, let's call our Self-Righteous Singer"Bob".
Porcupine Tree - Halo
Lyric Interpretation
[Verse]
God is in my fingers, God is in my head
God is in the trigger, God is in the lead
[Pre-Chorus]
God is freedom, God is truth
God is power and God is proof
God is fashion, God is fame
God gives meaning, God gives pain
[Chorus]
You can be right like me
With God in the hole you're a righteous soul
I got a halo round me, I got a halo round me
I'm not the same as you
Cos I've seen the light and I'm getting it high now
I got a halo round me, I got a halo round me
I got a halo round my head
[Verse 2]
God is on the cell phone, God is on the net
God is in the warning, God is in the threat
Verses
Both Verses 1 and 2 are what Bob believes about God. For example, he believes that god "Is In the Trigger" and "In The Lead" of the gun that he is firing. This is the reference to the religious wars that I was talking about.
The point that Steven Wilson [Song Writer] is trying to make, is that Bob is justifying all the killing by saying God wants him to do it.
Pre-Chorus
The Pre-Chorus is what Bob Is Saying about God.
He says that "God is Power and God is Proof" and "God Gives Meaning, God Gives Pain".
Bob is also representative of other religions such as Hindus, Christians, Or Catholics.
He is saying that god will give you all these great things like meaning, power, and fame, but god will also give you pain.
Chorus
The Chorus is where the real meat of the whole "Self-Righteous" thing comes in to play.
Bob says "With God In The Hole, You're a Righteous Soul".
The "Hole" could be referring to 2 things. It could mean the hole that most people have in their hearts (And most religious people would agree that God fills that hole).
It could also be referring to the saying "It's in the Hole", which means that it's under your control.
The Main Gist of the Chorus is, "I've got God under my control, and that makes me a saint. Heck, I'm Practically and Angel (The Halo!).".
I mean, to put it in Layman's terms that is...
Although Steven Wilson does his best not to give solid meanings to his songs, as to leave the meaning subjective and based on the listener, I hope you got the main meaning of "Halo" from this interpretation.