Jedi with purple lightsabers tend to be more balanced and sometimes use darker aspects of the force (not in his case). If you ever played KOTOR purple was the color that dark Jedi used (then again that game isn’t canon). Many Jedi that have purple lightsabers journeyed to the dark side at a time but came back and became even more light side force users.
The main reason mace windu had a purple lightsaber was because Samuel L. Jackson wanted a different colored lightsaber XD (sorry it’s so long, I tend to go on and on about these things lol)