Police officers absolutely can lie - everyone can. But when I'm trying to decide whom to believe, I find it helpful to compare what I know about the parties involved. So what do we have?
Michael Brown's friend...a self-admitted thief.
Michael Brown...also a thief, and one that thought nothing of assaulting a clerk in order to steal $49 worth of cigarillos.
Darren Wilson...a police officer on the force for six years with a clean record.
So, unless/until evidence is produced that shows that the police officer is lying, I'm going to believe him. If that evidence is produced, and ballistics should provide it, I'll be the first one saying that he should be charged and tried accordingly.
As to the officer not knowing that Michael Brown was a suspect in the theft, Michael Brown didn't necessarily know that. If he was willing to assault someone to steal in the first place, I don't believe it is unreasonable to surmise that he might be willing to assault someone (the police officer) in order to keep from being arrested for the theft and previous assault.