To see what you can do to try other options you can see previous comments that explain what to do, but If you haven't saved your save and persistent file before choosing, you can't change your choice.
[Spoilers]
For now there aren't many differences. The information I've gathered from other comments are the following. If you don't choose kill him or serve him during the lizard treasure storyline, in the orc's war storyline you can choose to serve him. If you choose to don't serve him, nothing special happens, while if you choose to serve him and after you confirm that you want to serve him or you change your mind once again, you should be able to have a sex scene with him that should be longer and have more scenes based on which enemies love you, that can involve Groth and that can kill you if your sanity is 40 or less. Plus there should be an option of asking to protect Groth linked to a Charisma test.