Leland didn't think that he'd find a way to grow closer with Sophie, but he apparently did. The man realized that it was only possible, somehow, by the loss of Sophie's husband. It was obvious that his gain was her loss. His happiness was her pain.
This reality made him feel awful. He was not responsible for Nicholas's death, but Leland knew he was only lucky because the other man, his competition, was gone. He could take the place by Sophie's side by tricking her into thinking he was marrying her for convenience.
He had also mated with her using the excuse that they needed to sleep together to make the marriage legally binding. Sometimes, he actually questioned himself if his reason to establish the mate bond was also just an excuse to be selfish, so he could truly mark her permanently as his.