The question is based on a crossword puzzle clue from a couple of weeks back. I forget the exact wording but the general idea was they were looking for the name of a Marvel comics supervillain, and the answer was “Ares”. The only supervillain I’m aware of named Ares was the one in the Wonder Woman movie, which is of course DC not Marvel. But I’m not a comic books geek so I don’t know. Did they botch the clue, or does Marvel also have an Ares?
Interesting note about the DC version that I saw on the Wikipedia page (which, I guess, is where I should have gone first… 
). Ares appeared in the first Wonder Woman comic in 1942, then in the second issue the name changed to Mars (the Roman version) and remained until 1987 when the series was rebooted and it went back to Ares.
And apparently the Marvel version started out as a supervillain but has become more of an anti-hero in recent years.
One of the goriest kills in Marvel comics was when the Sentry ripped Ares in half.
In the original myths, there wasn’t really a distinction between “good gods” and “evil gods”. They did sometimes face off in two or more factions (like in the Trojan War), but it wasn’t always the same factions. When they did so, Ares usually ended up on the side opposite the Greeks (because of course, Athena was always on Athens’ side, and you obviously can’t have both war gods on the same side because that’d be unfair), but that wouldn’t have stopped, say, a Greek soldier from praying to Ares for victory.