Mace Windu was one of the most powerful Jedi to ever serve in the Order. Before Anakin, Windu himself was the youngest ever Jedi to appointed to the Council and he quickly became renowned across the Galaxy. But most of the Jedi council privately held fears about Mace Windu and his tendency towards the Dark Side. Master Yoda even became deeply concerned himself that Windu would one day turn on the Jedi and fall to the darkness…
Yoda’s fear primarily stemmed from the fact that Windu went about his Jedi business in a way that was extremely agressive. Despite not quite being in line with the teachings of the Jedi, Windu was a man who would go for victory at all costs. Not to mention, he also believed that he was the chosen one before Anakin was brought to the Order. That is partially why their relationship was frosty for so long.
Because of this high held belief about himself, Windu was very harsh on himself and unfortunately others. Now on top of this, Windu admitted to deeply enjoying the feeling of beeing in battle. He loved the rush it gave him and the righteous justice he felt he was bringing to the Galaxy, through defending the weak. This was a huge problem though, because a Jedi really should not enjoy battle. Drawing one’s lightsaber is supposed to be a last resort for a Jedi and only in the most dire of circumstances… Once your lightsaber is drawn, most of your advantage is already gone…
This eventually deeply worried the council. They were concerned about how much he enjoyed fighting in battle, despite his great skill at it. In response to this, Windu literally admitted that he probably likes battle a bit too much, but refused to give up because of how good he was. Windu knew that he was on par with Master Yoda in terms of Lightsaber combat, so there was no chance he would give up. Yoda explained to the council that the path to the Dark side can begin with the smallest of steps. Even though it might not seem like a big deal, Yoda was deeply fearful that Windu’s enjoyment of battle would spiral out of control…
This was compounded by the fact that Mace Windu developed a brand new lightsaber form known as Vapaad, which directly chanelled the Dark Side of the force, using it against his enemy. Not only this, but the form chanelled Windu’s emotions into his fighting, amplifying his lust for battle… You can definitely see why Yoda was worried by this path Mace Windu was on. Vapaad was said by the council to pass through the penumbra of the dark side, broaching it’s cold embrace…