# That being the case, you will know ... "Then I will make

"Very well, I will go with you so that you will know ... Very well, I will make" 

# you will know what your servant can do

David probably wanted Achish to think that David would kill many Israelites, but this 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.

# make you

The word "you" refers to David and so is singular.

# bodyguard

a person who protects another person