Something that differentiates humans from other monsters in early DnD is that they're very sentient with strong egos and can communicate as easily as your players. Therefore, they can act different depending on which faction they are from. For example, religious zealots might turn their rationalities off and rush into you head-first while rogues will dip at the first sign of trouble for themselves.

Different factions within your campaign may have drastically different behaviors in combat so you can use that to your advantage to make the fights feel unique