1)
Why did David want to stay elsewhere?
1.
Radak: He wanted that (a) Achish not know that David wipes out other places, and (b) he and his men will have their own city.
2)
How did David explain to Achish why he does not want to stay in the capitol?
1.
Rashi: I will be a burden to you. Radak - he made Achish think so. Refer to 27:5:1:1.
2.
Malbim: Perhaps you want me to be in the capitol lest I rebel against you, and your legions are there - please do not suspect me! If Pelishtim fear that I be with them not in the capitol - give to us our own place! If you fear to give to me a walled city, lest I fortify myself there - you can give to me an open city!
