What does " before going on a journey for revenge, dig two graves" mean?

One grave for your intended target, one for yourself. If you are sufficiently intent on revenge, you are likely to destroy yourself in the process.

I think the author meant the consequences of "hot" revenge. Another author states that "revenge is a dish best served cold": well planned and executed, savored in private.

- .--

Dig one for your enemy and a second for yourself.

The grave you dig for yourself means that even though you may kill your enemy and survive you will always regret what you have done and wish you had not.... you will have buried something of yourself alongside your enemy; something very important, like....your self respect and their death will haunt you throughout your life..

Dig one grave for yourself. Such a journey is all consuming.

One for whomever you're taking revenge upon and the other for yourself I guess.

Because vengeance belongs to GOD, and everyone practising it kills his object and himself as vengeance is a mortal sin.