Evil doesn’t mean they’re lying all the time though, and if we’re referring back to other external references (“satanic”) then there’s a lot of stuff where bound demons are compelled to speak truthfully, and not lie, as part of their bindings.
The quest doesn’t specify the details of the contract, we can’t say for certain, but if Kanrethad bound this doomguard to answer his questions - which he seems to have done so - it would be reasonable there’d be a truth-telling clause in there.
They did so in search of World Souls. Aggramar choosing to intervene on Draenor where there wasn’t a world soul is called out as unusual. And a bunch of those planets and life forms were burned in the Makers’ fire, per Algalon, to purge the corruption of the Old Gods. Again, they don’t treat World Souls the same way.
The act of creating life isn’t a moral good all on its own, so I can’t say I care for that explanation, especially if it’s to help nurture the world souls and help them grow. Means to an end.
You’re completely wrong about the “30 years” stuff too, sorry. The titans were barely touched on, if at all, in WC1+2. They might have been creator myths or something.
I don’t remember them getting anything notable in WC3 (maybe in the manual), but they only really started showing up in WoW from vanilla onwards. We had vague stuff about them in vanilla, not a whole lot, little to speak of in TBC since we were on Draenor, and then Wrath introduced Algalon with his deal. That gives like ~3 years of vagueries before we started dealing with the Titan lore properly.
We cannot say with any straight face that this latest information is a shift in the Titan’s portrayal. It is a continuance of it. They are not malicious, but they don’t care about mortals, not in any grand scheme, and if it means maintaining a world soul they’ll sacrifice a planet - whether their lives or their memories - to do so.