I prefer to presume that people are telling the truth, until we're given reason to believe they're lying.
Why? Because at some point, you could stop trusting anything that people say, and call it all lies.
So if we're given reason to believe a character is lying (through exposition such as flashbacks, or testimony from other characters), then I'll accept that someone is lying. Ben, for instance, has lied so often in the past that we know not to trust him.
Spoiler
For that matter, other than the fact that the unLocke killed Jacob's bodyguards, we really don't have evidence about who is "good" and who is "bad", or what their underlying motivations are. At the end of the day, we might end up sympathizing more with Smokey.