Why did Toby from Pretty Little Liars kill himself?

Answer 1

Yes, because Emily finds out about his secret, Allison saw him touching Jenna inappropriately when she was going to set off the fire cracker and accidentally blinded Jenna. She told him if he kept his secret he had to keep hers.

Answer 2

Yes, after Emily says "I know what you did to her" thinking he killed Ali, he thought she was talking about Jenna, his stepsister that he used to touch. Since she was so scared, she had locked him out while he was standing in the pouring rain. He took off and in the morning, a biker stops at their house saying he found a blue faced boy dead in the woods. The cops came, realizing he had taken this medication (i forgot what it was) but he took this pill that would kill himself. He leaves Emily a note, saying how he couldnt live with his secret since she already knew and it ended himself. But really Emily didn't know, until Spencer then confessed to her that the night of The Jenna Thing happened Ali had seen Toby in his tree house touching Jenna, she was so shocked she knocked the firework out of its place and it hit Jenna. Little did the girls know, it was all Jenna and Ali's plan to get Toby out because of what he was doing to her. She told Ali because she thought she would understand since Ali said she had sibling problems too.

Hope you understand now!!
In the book they think that he did.

Answer 3

he kills himself because when Emily told him about what she did to his step sister (jenna) he thought about what he did to Jenna and feels guilty and he kills himself

Answer 4

He commits suicide because he feels bad for what he did to Jenna (his step sister).