[spoilers] Lucas. Just Lucas.

[Saerwen]

I'm afraid I am not as good as expressing myself as others have so eloquently with their marvellous posts but here goes.

I think Lucas had a subconsious grudge against Harry. He wanted Harry to have the same experience as him, feel what he felt and Albany fulfilled that need. If Lucas had received the right support on his return he may have been mentally stronger to resist what Vaughan was doing instead of it taking him over, and his need to get away.
I think Lucas was exceptionally stressed and with Vaughan giving him the case tipped him over and this other personality of John surfaced. Lucas still managed through the turmoil in the last ep not to kill anyone, he went out of his way to keep his team safe. I don't think Lucas had any intention of killing Harry just frighten him as he did the others.
I had a friend who had a personality disorder and they would change into someone complete different, if was something you had to witness to believe, so entirely possible for a inner conflict of two personalities and it just takes something to be the last straw and it can be something you are not aware of but your subconsious registers it.
It's like Lucas couldn't get away due to his loyalty but John had not such restraints and could do anything more of less.

It was very convenient for Vaughan to have on his person for x amount of years a suitcase full of stuff relating to Lucas, who on earth hangs onto stuff that amount of time that's not yours unless you have some long term plan in the pipeline.

I think a plot holes discussion thread would be good, as there are so many of them.