Resch fears that he may be an android because he loves to kill. Here is a the character analysis of Resch that you will find on the eNotes link below, as well as that of all the other characters:
Phil Resch is one of the bounty hunters working for the false policeman Garland. After revealing his own identity, Garland tells Deckard that Resch is an android because he hopes that Deckard will kill Resch—thus making him guilty of murder and putting him out of commission.
The problem with Resch, according to Deckard, is that he enjoys killing too much. Disturbed by Garland's real identity, Resch starts to wonder if he too might be an android, despite the fact that he loves his pet squirrel. Resch demands that Deckard test him—and he passes. The contrast between Resch and himself leads Deckard to reconsider his line of work.
Resch kills two of the androids on the list, but Deckard will take the credit, since Resch is now a fraudulent bounty hunter who just killed his boss. Deckard then confides his doubts about the job. Resch proposes an easy solution, which he himself used—sleep with Rachael. Deckard follows this advice, but it has different effects on him than he expects.