Depression? Some of the other clones have suffered from physical maladies, so it's entirely possible that Beth's nurture, for whatever reason, could have caused her to develop a mental illness. Or reactive depression from finding out everything she knew was a lie, the pressure from the shooting, and that the man she was dedicating her life to was betraying her and really didn't seem to care about her.
I've been thinking about Beth's suicide. In episode 2, Alison doesn't believe Sarah's story that Beth killed herself ("nope, that's impossible"). And then in the finale, Art tells Sarah that he "didn't see it coming." On one of the OB podcasts (tatianaiseveryone... I highly recommend it!), someone observed that it was strange that Beth would off herself by jumping in front of a train, when she could have easily swallowed a handful of her ample stash of pills, or just use Paul's gun.
And then I look at the woman who jumped. There was something all too elegant about her. When you first watch the opening scene, the last thing you think is woman police officer.
I'm going to go out on a limb here, and propose that the sister who died at Huxley station was not Beth Childs. I think Beth is still alive. I think she was in cahoots with another sister who stood in for her at the police station while the real Beth went deep undercover. Maybe one of the Euroclones faked her death and escaped to Canada to work with Beth. It was this other sister who had severe depression or psychosis, which ultimately led to her suicide. Maybe Beth and this other sister caught wind of a killer coming to Canada, and when this sister saw Sarah on the platform, she thought Sarah was killer clone.
Far fetched? Probably. But we know that Beth was not telling Alison and Cosima everything. She didn't tell them about Maggie Chen's connection to the Prelethians Hence Cosima's "Holy watershed" exclamation when Sarah connected Helena's flying fish blade to the brand on Maggie's neck.