Cicada also left Ralph and Cisco alive, which could indicate that he was shocked by Nora’s appearance rather than suddenly filled with mercy to spare her dad.
He wouldn’t have been killing Ralph and Cisco in front of their children!
After all, the Flash certainly won’t be the only meta on Cicada’s kill list to have a family, and he wouldn’t be very effective at exterminating metahumans if he spares every parent.