[Voralphion]

In the Fall of Gondolin, it says that those lead by Maeglin did in fact kill their own kin, although it doesn't say why they were traitors. Perhaps they too like Maegin were captured and released by Morgoth and were under his command, or it could be that they were offered something they desired, be it power, riches or a woman, like Maeglin was. Perhaps they just didn't like Turgon and wanted to see his city fall.