Romeo says he has come for  poison to kill himself because he has found out that Juliet had died.

Because he could go to jail for selling it.  And that Romeo could die from using it

‘the world is not thy friend’ means the world is not being fair towards Romeo. Romeo is angry with everyone because Juliet has died.

because the apothecary is poor

The poison could kill 20 people

Money is worse than death and poison itself.

Friar Lawrence asked for a messenger.