Shakespeare killed them because the play was meant to be a tragedy, and Shakespearean tragedies always mean at least several of the main characters end up dead.
There are several sub-stories at work in Romeo and Juliet. Of course the one about immature teenage love/lust is one of them. But so is the morality play of two opposing sides that hate each other so much that they can't allow any intermingling or peace overtures, because it would be a perceived weakness or loss of pride. This is a common story thread, no matter if it's the traditional Romeo and Juliet, the musical adaptation West Side Story (two opposing street gangs, each of differing ethnicities), or even a "Hatfield & McCoy" type of Western.
They were immature teenagers who had limited ability to think ahead.
I have to wonder... if Romeo had never come along, would Juliet have meekly married Count Paris, or would she have refused? Remember, she was only 14 years old. To us that makes her a child, but back then a 14-year-old female was considered old enough to marry and have babies.