In my opinion, though it brings merit to set up your own alter but the fact that this person took its merits away from many others, this causes it to be a sin. I believe that this statue was there to benefits others, when this person took it with him, it took away the many chances of others to sprout their seed of dharma. Although this also depends on the persons motivation, whether he took it away to set up another alter in a more public, open area to bring this statue to many others OR he stole it and decided to sell it or keep it all to himself. Even if he wanted it for the right reasons and have thoughts of bringing it home with him, he could just snap a picture, print it out and stick it on his alter instead of taking the whole statue with him.