He definitely has his share of secrets and is great at hiding them
He fathered Jason DiLaurentis which I think is still unknown to Mr. D (right?)
All signs point to him as the suspect for killing Mrs. D
Melissa is working along side him with Mrs. Hastings and Spencer on the opposing side
He practically sent his (soon-to-be-ex) wife to the spa, which was probably a means of getting her to stay quiet since she wasn't stable after Mrs. D's body was found in her backyard
We find out that Melissa and Mr. H were lying where they were the night of her murder
When Mrs. H tells Spence that, she said, "We promised we would never involve you and Melissa in all of the......." and she cuts herself off. I feel like the Hastings have secrets that could probably send the whole family to jail (at least the adults, not sure about Spencer) and Mr & Mrs Hastings made a promise to eachother that they would never involve their children in whatever family secrets/hidden agenda they may have. Because Mr. H broke that promise to his wife, she now knows that she can no longer trust him because he would basically risk his marriage AGAIN (first time, sleeping with Mrs. D) to follow through with whatever his next plan is.