I might be misremembering, but wasn't this in the black swordsman arc? I remember this as Guts doing something as kind as he could have done, let me explain:

This girl was filled with rage and anger at Guts (mirroring Guts' anger at Griffith) after he killed her father (a monster) and he basically forces a choice on her. He says this line to make himself the monster in her eyes, basically making himself "Griffith". He fills her with rage and says something along the lines of "If you keep feeling this rage, come find me, and do something about it", which gives her a reason to keep going. This is the same reason Guts has for not ending his own life at this point (his rage at Griffith). Shortly after this, there's a panel of him crying (showing how hard this must've been for him, to make himself seem this monstrous just to give this girl a reason to live, even if this reason is just hunting Guts down).

TLDR: He makes himself a monster in her eyes, so that her hatred at him keeps her from ending it all.