I would like to sympathize. I remember my first love. Thought I would die if we had to part. It's too bad that the parents didn't arrange for you to be the bridegroom, but they didn't.
Where do you live? U.S., Canada, Europe? Where do her parents live - still in Pakistan, or in the West? Are they Muslims (I presume, since Pakistan is mostly a Muslim country)? Are you both the same faith?
If you were to try talking to your girlfriend's family, would this be in person or email; phone; letter?
Are you Pakistani or East Indian, from someplace at least familiar to the girl's family?
I'm asking all these questions because the more similarity you have with each other, the better chance your relationship has of enduring. Also, if you are 20 years old, she must be around your age.
WHen you wrote "her parents would kill both of us" was that an exaggeration, or is that likely in her culture? If it IS likely, and you really love her sooo much, you will prove that you love her by breaking off completely and cleanly, no emails, text messages, phone calls or anything that might compromise her safety. Her life (and yours) are irreplaceable.
Are you having sex together? If so, you could really be endangering her life. In cultures where arranged marriages are the norm, virginity can mean life or death for the bride. Please, if you haven't had intercourse, don't.
Last, I think the statistics on success of arranged marriages are about the same or higher than love matches. Once the marriage actually happens, it could actually work out well for your girlfriend. I know you don't want to think about that, your emotions are so high. If you can be selfless about her happiness for her lifetime, bowing out and breaking up could be the best thing you can do for her.
If you were to marry against her family's wishes, and if they cut her off, she will be lonely throughout her life. If your "they would kill both of us" is literal, that might not be a very long time. If your "they would kill both of us" is an exaggeration, it's still possible that they will snub the births of your children, refuse to invite you to dinner or return your phone calls, or even answer the door if you knock on it.
Your girlfriend might not be able to stand being cut off from her family. The strain of that rejection will affect your love for each other sooner or later.
That's my suggestion.