The trick is not to TRY to roleplay him as scary. Roleplay him as a normal guy - who is so far above the level of the PCs that he isn't remotely concerned about them. As others mentioned, his actions speak louder than his words. have him standing by while something else (zombies, wolves, etc) attack the party - and if they try to take the fight to him, he just fireballs them all. Like @
pukunui, I had him show up at the father's burial as well - where he one-shotted a player with an unarmed strike; he literally slapped the PC dead in one hit. That will scare the bejeebers out of the players way more than cheesy tag lines.
When he does speak, I see it more as a condescending sort of thing - "Welcome to my home, enjoy yourselves, but don't make too much of a nuisance of yourselves or this game will end early..." They should encounter him several times before the final battle. At first, he can be nice and welcoming - after all, the PCs are there at his whim. Hhe toys with them a bit, but nothing too serious, just testing the waters. As they rise in level and get more involved in righting wrongs in the valley, maybe he issues a "stern warning" to keep their nose out of his business, and then attacks for a few rounds just to bloody them a bit before walking away unconcerned. It's only once they get the artifacts and a higher level that he really starts to take them seriously - at which point he invites them to his castle and messes with them till they go nuts or die.