How can I stop this guy from killing me?

Solution 1:

Unfortunately, there's no way of resetting the Crestfallen Warrior (short of starting over) after he's become hostile. See below for a way to actually calm him down. (I forgot all about Oswald!)

However, you are not penalized in any way for killing him/causing him to die. According to the Wikidot:

Be wary about accidentally making him hostile the first time you meet him, as he'll attack you as soon as you respawn at the bonfire.

It goes on to say:

As he doesn't drop any item nor does he have any effect on other NPC's story, you don't need to worry about killing him by kicking him off cliff as his "full reward" is only his souls.

Which is to say, there are no penalties for killing him, and you can snag a quick 1000 souls in the process!

Solution 2:

You can pay Oswald of Carim in the Bell Tower to absolve your sins. It will cost 500 times your soul level to absolve your sins, but it will reset all angry NPCs to neutral.

Solution 3:

Just roll off the hill and he should follow you to his and your death.

Solution 4:

A quick method of dispatching the Crestfallen Warrior is derived from his pretty slow attacks. Try meeting his attacks with a parry and riposte, or dodging by rolling. You can also circle him with minor difficulty and then backstab him... these techniques come in VERY handy for most small enemies in the game so this battle is a good training ground for your Parry/Ripostes and your backstabs. Just remember you can always retrieve your souls before he kills you and you wont lose a thing providing that you are hollowed.

Oh and another little tip: at low levels try not to attack NPC's although they look helpless and pathetic (most of them), they are hard as nails to defeat!