The phrases "That being the case" and "Then" show that the speaker agrees with what the other person has just said. Alternate translation: "Very well, I will go with you so that you will know ... Very well, I will make" (See: [[rc://en/ta/man/jit/writing-connectingwords]])
David probably wanted Achish to think that David would kill many Israelites, but "what your servant can do" could also mean that David was planning to kill Philistines instead. Try to translate so that the reader can see both of these possible meanings.
David speaks as if he is another person so that Achish will think David respects him. Alternate translation: "I" (See: [[rc://en/ta/man/jit/figs-pronouns]])