Aku doesn’t use torture a whole lot but when he does, you DO NOT want to be on the receiving end of it. I think there’s a difference between sending someone to the Pit of Hate for punishment and what he would call torture. I can’t say for sure what happens in the Pit of Hate but it’s hinted at being survivable where the person who enters can return from it alive somehow once their punishment has been served. 

I think Aku tortures people by granting them immortality (or conditional immortality) and then trapping them in a place forever. Like with the man in the volcano, he was granted partial immortality so that he couldn’t die (until someone like Jack came along) and was kept from entering Valhalla. This was the best way he could think of to punish someone who wasn’t afraid to die. 

It’s highly probable that he’s torturing Jack’s father in a similar way where the Emperor can’t die but he’s constantly consumed in flames and is in a world of relentless agony. If that’s true, he hates Jack’s father more than he hates Jack himself since he only wanted to kill Jack in S5 rather than burn him forever. It seemed like at that point, he saw Jack as a major pest and just wanted the peace of mind of having him dead.

That said, I don’t think Aku is particularly sadistic. I’d rate him maybe a 6 out of 10 on the Sadism scale since he doesn’t seem to get off on torture. If you’re unlucky enough to be someone like Jack’s father or the lava man, Aku probably wouldn’t stick around to adore your suffering. He doesn’t really have the patience or attention span for prolonged poking and prodding. He’s more the type to just laugh at your pain for a while and then act like he forgot about you.