The way I read the rules regarding alien movement is that an alien can to a hex other than aa area center hex only if the center hex is already occupied or it is redeploying to a hex to become adjacent to an agent. Other that these two situations, an alien must stop its movement in an area center hex. Is this correct?
Mostly; technically, the alien can redeploy if the player controlling them thinks that it is in the alien's best interest.
Practically, this rarely comes up. Typically the only reason to redeploy aside from getting adjacent to an agent is to maintain LOS.
For instance, if a Xeno-beta is at range 3 with LOS to an agent, it will move 1 area closer and then fire on the agent. Sometimes the movement can result in it losing LOS (it fires on the agent either way, but if it can maintain LOS by redeploying, it should).