Evil =/= Antagonist
An Antagonist would be a force in which for PCs to organize and act against, basically each Antagonist is like an individual plot to deal with. Which is why Elf is controlling how many and what PCs are Antagonists.
To add to this, I'll again use Tom as an example (sorry for this
). He would again, normally be in my antagonist slot, but since he has the potential to actually switch sides and work with any other PCs instead of against them all, he's in a regular slot. His alignment is actually true neutral either way.
So like Knick said, you can have an antagonist that is actually Chaotic Good and a regular PC that's Lawful Evil. It has to do with their role rather than alignment. Though it's likely that most antagonists will lean towards the evil side of things, but it's not a requirement, just a trend.