Spoiler Alert - if you haven't watched the most up to date episode.
As we have seen from the last couple of episodes with Beth flash backs she, with the help of MK or Mika, had worked out quite a bit of the clone stuff and the connection to neolutionists.
In the most recent flash back Beth ended up in the clinic threatening Prof Duncan (female), nobody had made it that far before in terms of discovery. Beth had been primed by the Asian lady (whose name escapes me but was called 'the engineer') who supposedly worked with Prof Duncan, to believe that killing Prof Duncan would be the end to all the clones' troubles.
Asian lady was basically using Beth to do her dirty work in trying to oust Prof Duncan. But Beth didn't kill Prof Duncan after she said 'but I love you all'.
When Beth ran out of the building after NOT killing Prof Duncan she was in a car with Asian lady who is not happy that Beth didn't do it. Asian lady tells Beth that she knows too much and therefore they (being all the clones) will have to be killed. If you recall, there previously was mention of Helsinki which is where all the clones were wiped out, except MK who escaped.
It was only Beth that who had found out so much, so to protect the other clones she killed herself. If she hadn't then all of the clones would have been wiped out.
In other words Beth sacrificed herself so that the other clones (sestras) wouldn't be killed off.
I'm glad they explained why Beth did what she did. It never sat true with me that Beth randomly committed suicide, but this explanation shows that she did it to save the others.