Remember season 4 in the parent show where Sylar spent half the season inside Matt's head and tormenting him about his family issues and restraining his ability use? Some of his corrupting influence lingered inside about using and abusing his ability. Factor in the fact he already had issues about his ability already and with his family life in complete tatters, Matt had nothing but his ability left. So why not go ahead and use it? When Erica presented him with an opportunity to hopefully fix his family life, of course he'd jump at the chance.
The premiere reveals that Claire Bennet was killed sometime after the end of Heroes. You know, the girl who was basically invincible? So much so that she recovered from having Sylar rip her head open?. Yeah, something killed her. What the hell is that powerful?
An eclipse, specifically caused by Phoebe's Umbrakinesis; it has a broad-spectrum depowering effect for many Evos. It took away her healing abilities and she died in childbirth.
The real reason turns out to be Claire's son, who inadvertently stole her ability and left her unable to heal, resulting in her Death by Childbirth.
