I guess with Aku I just get the sense that time has done just as much of a number on him as it has on Jack. The second episode showed pretty much the extent of how "broken" a character like him could be to the point that he's become an apathetic shell. Even when he kills an entire armada and the Scotsman he seems completely ambivalent to it, and I think the latest episode is the only one where he's finally gotten some of his spark back.
To me I think the intimidation attempts drawing from different sources - mostly how despite coming across as apathetic and unassuming he still holds dominance over space/time with an iron fist, and that just adds a level of unpredictability; such as when you can kinda tell that he's gonna vaporize the Scotsman and yet his disposition has you hoping he doesn't. Then there's also the latest episode where his spitefulness is all the more terrifying than usual because he's exploiting a more heartfelt than usual bond between two characters that's been cultivating throughout the entire series. In a way I think circumstances in this series has allowed for Aku to be intimidating but usually not through him directly - more so because of him specifically showing up for moments where you know his presence won't be good news.
It's like that gleeful expression he has when he shows up right behind Ashi. It's gonna end poorly, and the fact that he's fully aware of that is what makes him scary in the moment.
Which reminds me that shot where he shows up in front of Jack and Ashi was legitimately kind of creepy. The ominous silhouette, especially knowing that this was looking to be the final confrontation, was all kinds of foreboding.
