I have no skin in this game, but this thread is the first time I'm learning there was a 3rd victim, and I've been checking news sites twice a day for the last few days.
It was very stupid of the authorities to downplay the third victim as it lends itself to specifically these kinds of conspiracy theories.
It's perfectly understandable that a radicalised, hate filled man could attack someone known to them, and then feel emboldened to carry out a terrorist attack. One does not occlude the other, it supports it.
Just as the Southport monster practiced terrorising his family before he went on to commit a gender-based terrorism attack.