I understand why from a plot perspective, but nitpicking at his decisions... Telling lydia that he poisoned her was a stupid move.
He knew that she won't instantly die, she certainly has enough time to call in a revenge kill on walt's family.
If Lydia even had the connections anymore to call out a hit then she would have been more likely to do it after realizing that Jack's gang was wiped out. Because that threat was the only thing keeping Skyler from talking.
Now that she knows she's been poisoned there's a good chance she'll call for an ambulance, and it's possible that she'll survive the poisoning given proper treatment. It's not clear if this was the show's intentions though, they made it seem like ricin poisoning was guaranteed to be fatal. If she does survive then the police will probably find enough evidence related to the poison to try her. It could be she realizes this in advance and just lets the ricin kill her.