I'd say it's an impulse control problem inasmuch as he decides to "see who he can find", and then, if the opportunity presents itself, he strikes. Very different from some other serial killers who create the opportunity by entering a dwelling (Nightstalker), setting up a ruse to lure someone (Bundy), or grooming someone over time (innumerable pedophiles). From what I can see, he doesn't create the situation; rather, he takes advantage of the situation presented to him (the original conviction; Amber; Chelsea; and who knows who else). All were in a place where he saw the opportunity, and he took it...while he placed himself in the area of opportunity (by driving by school, hanging at the park), he didn't break and enter, nor elaborately plan a kidnapping, nor groom his victims. Those take a more organized, less impulsive personality than he seems to have.
I don't think the defense will try to use impulsivity as a defense, and if they do, without an organic issue in his frontal lobes, it will fail miserably. We all 'want' to do things, and we all have impulsive things happen (how'd that chocolate get into my shopping cart??), but the level of his impulsivity is not going to get him out of this.
And if the prosecution is good, they will use this as a bolster in his MO...opportunity presents, he strikes. That leaves very little time to 'prevent' things by "thinking about it", and presents a clear and present danger to the public at large whenever he's free.
Hope that helps clarify my thoughts on it...
Best-
Herding Cats