Giving a Big Bad a Freudian Excuse

So story features a setting where nearly everyone is The Ageless and the main conflict arises when the 'villians' attempt to render everyone except those who they believe are most worthy into 'mortals' At this point I'm not sure if I want the Big Bad to actually be a true 'character' or to lead the other villians as an Offstage Villiany. They manage to get a good deal of followers to work for them, but I'm trying to think of other reasons they would want to become allies with him, this is what I have so far

• They believe in Social Darwinism or Kill the Poor
• They believe that the evil army poses a genuine threat and if they become part of it, they will be able to retain their immortality
• They had a Dead Little Sister whose death was caused by a lack of resources, that could have been avoided if other 'unworthies' hadn't used them

I assume that by "mortal", you mean "aging"? The Ageless is already mortal.

In my settings the 'immortals' cannot die unless it is self inflicted or they run out of 'magical energy' and die, but the conditions are pretty specific so it rarely happens normally.

Well, Dead Little Sister (not Dead Little Sister; that's something altogether different) would work as a Freudian Excuse to me - at least, for the top villain.

But do you think the other reasons are realistic for someone to be willing to pull a Heel–Face Turn?

I think you want Face–Heel Turn here (Heel–Face Turn is "bad guy turning good").

Aye, I would consider these reasons to be realistic. That said, I would use different reasons for different followers.