Let's start with two quick lists: the first provides good traits to give a villain when you want to show they're villainous. The other list provides bad traits to give a villain when you want to show they're villainous. 

Good traits to show someone is a villain:
Cruelty or abusiveness
Casual or wanton destructiveness
Corrupted motives 

Bad traits to show someone is a villain:
Ugliness or disfigurement (especially facial disfigurement)
Exceptionally overweight or dramatically underweight
Non-heterosexual (whether gay, bi, asexual, or other in a way presented as "abnormal")
They've been tortured or abused
Mental health issues, particularly when given pejorative terms like "madman" or "lunatic"

The difference between these lists is hopefully pretty apparent. The latter consists of a lot of tired tropes, as well as features that alienate and offend real people by vilifying aspects that are part of their nature or beyond their control. Most traits on the second list are entirely physical, and identifiable even if the person is standing alone, doing nothing, in a neutral space.

The first list are all actions, or traits that require actions to demonstrate. Your villains shouldn't just be anything. They should do things to show they're the villains. Often, these are straight-up villainous things: a town mayor caught sacrificing people on a bloodstained altar in "Room 12B: Temple to Orcus" is the bad guy, no problem. But the same mayor might first be introduced to the PCs as aggressively bullying his subordinates, ignoring pleas for help, or hoarding luxuries while his town suffers from want. A villain should take actions that hint at his evil traits; this communicates villainy to your players far more effectively than, for example, saying the mayor is "an ugly fat man" or "scarred and twitchy."