The Greek term translated here as respect
is the common word for fear.
The question is, To whom (or to what) is this fear directed? If it is to be directed to the master, it amounts to an instruction to the servant to cower before him—which he unavoidably would if the master were unjust
(think of Potiphar's wife in relation to Joseph). Yet in such a case we cannot imagine how the Holy Spirit would inspire the apostle to instruct a slave yet to respect
him. So it’s worth noting that when Peter elsewhere uses the word fear
he is referring to the right attitude toward God (1 Peter 1:17; 1 Peter 2:17 (previous verse!); 1 Peter 3:2, 1 Peter 3:15). Deep respect for the sovereign God who leads your life means that you accept what he in wisdom puts on your path and entrust your safety to him.
18 Servants, be subject to your masters with all respect, not only to the good and gentle but also to the unjust.