I have zero sympathy for Ian Johnston and, yes, I do think his behaviour and actions contributed in a small way to the overall plot.
The blame for the actual murder lies solely with the murderers but Ian Johnston brought Williams into their lives and continued contact with Williams up until the eve of the murder which would have confirmed to her that he would be hers if only she remove Sadie.
Ian Johnston's woeful "There but for the Grace of God go most men, because they would have behaved like me." was utterly disgraceful and his soap opera type crying where there are no tears was pathetic, he's looking for the sympathy vote and, in my opinion, doesn't deserve a shred of it.