What do red names mean?
Solution 1:
The red names are players who have been marked as a target. In Operation Bloodhound there are two missions that do this, one is Sergei while the other Jackson. The game randomly picks a (human controlled) player on the appropriate team and sets their name to Sergei/Jackson and keeps selecting a new player each time they die until all the players on the other team who need the challenge have killed him.
If you do not have Operation Bloodhound or you are not currently playing that mission the name change has no effect on gameplay.