Lot's of reasons:
The anti-Christ or their representatives convince others that they are totally not evil, simply misunderstood or misrepresented by the powers that be that are the "real evil".
The "good-guys" have a puritanical belief in eternal damnation of sinners that drives people away, why not worship the anti-Christ if you're already being sent to hell for some trivial reason like eating on the wrong day or accidentally lying a few times.
They are simply A-moral and believe that there's nothing wrong with turning to the "dark side". while this is certainly cynical I have met people with similar mindsets.
Good old fashioned mind-control or "corruption" think the dark-side in star wars, the one ring from LOTR or chaos from WH40K.
the anti-Christ claims to represent evils which are while enjoyable ultimately necessary for universal balance. For example many people believe that god allows evil to exist so that it might better serve help us appreciate the light perhaps if people in your world also see things this way perhaps they will support this anti-Christ as a necessary evil. Alternatively it could be a literal cosmic balance that must be preserved for existence to work.
They wish to use anti-Christs powers for good, foolishly assuming they can harness them for positive change rather than getting wrapped up in the whole "soul loosing" thing. This would probably work best if the "Christ-Christ" is distant and not a lot of help, and if people have the capacity for idiocy (just checked yep people are still often stupid).
if you would like any elaboration on any of these points just comment below.