In the Star Wars: Republic series, multiple Jedi have red lightsabers. Given the association between red lightsabers and the Sith, why is this? I'm looking for both the in-universe and out-of-universe reasons.
I haven't heard of the Star Wars: Republic series, but I do know that the color of the lightsaber doesn't necessarily denote whether you are a Dark side user or a Light side user. It depends on you. A Jedi could have a red lightsaber. That doesn't mean he is a Dark side user. He could have gotten it from a Sith he defeated in battle if the Sith was using a red saber. Just like you can take a blue lightsaber even if you are evil, like with General Grievous. Also, artificial kyber crystals usually are red and produced a red colored lightsaber blade, but a Jedi could use an artificial kyber crystal if they wanted, but still be a Light side user.
The jedi believed that the sith were defeated and the last of the dark jedi had fallen centuries ago. Some jedi were beginning to carry red lightsabers because the sith and dark jedi were defeated, and many jedi thought it was accpetable to wield red blades, as depicted in Star Wars: Republic. Star Wars: # 14 Emissaries to Malastare 2 of 6.