So you think you know more than Kevin Reddington?
If there were any evidence that Patrick Clancy had a role in the deaths of his three children, Reddington would use it.
A lawyer who has spent his entire career identifying the strongest available defense does not concede the act if there could be any chance of tying Patrick to it.
Reddington has never left evidence on the table.
His job is to work for the best outcome for his client, Lindsay. His job is not to present a case against Patrick. Which would fall on the prosecutor, and probably still should after they realize how badly they fucked this up.