Hello there,
so i have a Female Dalish Elf who is romanced with Leliana and when Morrigan comes to my room before the final battle against Archdemon I persuaded Alistair that he must have a baby with Morrigan so that we could both survive. Also i made him a king (not marrying him with Anora 'cause i don't like her) and when I choose at Keep that he killed Archdemon it pops up that Morrigan can not have a child with him and so that he must die against Archdemon.
So my question is this: WHY?
All I want is to show Ferelden that Alistair is a great king and when the next king of Ferelden kills the Archdemon and end the Blight that will help to boost in citizen eyes to the maximum level so that they must cheer for him that he is the great king who defends his own kingdom. I only want Alistair to believe in himself that he is a great choice for Ferelden and not Anora so I let him live, kill Loghain in a duel and made him hardened at the end of his Quest.
Sorry for my mistakes in this. I'm not so skillfull in English so please forgive me.
First off, your English is fine and better than plenty of native speakers on the internet, so please don't worry about it.
Secondly, to clarify and give a bit more detail on the above answers, if you perform the Dark Ritual with Morrigan, the Warden will automatically take the killing blow. There is no conversation about it and no ability to choose someone else. As to why this was done; I don't know. Perhaps to make sure the Warden was the hero, perhaps for ease. I agree that it would be cool to still decide so Alistair or even Loghain could get the glory. But really the emotional weight was the requirement to sacrifice someone so they probably decided that without that sacrifice, the weight of the decision was gone.
On a Keep note, make sure you set that Morrigan had the Old God Baby first.