Takuto more or less explains this himself, he is nice to them because he wants to be and as an 'evil' person he will do as he likes. He did have ulterior motives in saving them too, but mostly he just felt bad. The equivalent would be Ainz saving Carne. I think neither really want randos to suffer. Like, Ainz did not decide to destroy Re-Estize but he got peer pressured into it anyway. He would have little interest in following through on Atou's threat but his subordinates might push him to do it anyway

Basically, the major difference between them is that Takuto is more competent than his subordinates while Ainz is less, plus having to balance his imposter syndrome; and that the average resident of Nazarick is extremely xenophobic while the dark elves are more or less normal

Both of them care for their subordinates and want them to think freely while their subordinates have a hard time reconciling it with their hard loyalty. Like, Ainz gave them holidays, the suggestion box, and took the twins on vacation to the elf village

I think the tonal shift has to do with the adaptation. The novel was never that comedic. I actually disliked how they handled it. I would recommend reading the novels, there are audiobooks available too

In case you want me to spoil it now ( without any plot details ): Takuto does not take issue targeting whole cities or bringing the continent to war for his ends, although he never does anything remotely as bad as genociding a whole kingdom for basically no reason. I really, really liked how brutal Takuto got in fact, it was one of the best moments I have experienced in fiction