WARNING: The following contains spoilers from Season 5, Episode 10 of My Hero Academia, "The Which Is Inherited," now streaming on Funimation, Hulu, and Crunchyroll.
In Episode 10 of My Hero Academia's fifth season, "That Which Is Inherited," Deku gets seriously mad when Monoma pushes the All Might button to provoke him. The One For All successor succumbs to his anger for the briefest of moments, causing all hell to break loose. His emotions cause a new Quirk to manifest, resulting in such a dangerous scene that one of its Predecessors stops in for a chat.
Anger is a normal human emotion, something Deku must come to terms with. This doesn't have to hinder a hero -- as Bakugo demonstrates, considering his default emotion is anger. Even All Might exhibited rage on the battlefield when Bakugo slipped through his fingers, but it did not compromise his judgment or the mission.
The new Blackwhip Quirk is one of six powers that Deku learns are hidden within One For All. This new ability cannot be used thoughtlessly, especially after it's been cultivated for so long and has grown stronger than ever. The Blackwhip Quirk's original user -- the fifth inheritor of One For All -- tells Deku that anger is the source of power, but power responds to anger in turn. If young Midoriya loses his temper, he will lose control of his power too.
Deku's emotions often serve him well in a world that is becoming increasingly morally grey. However, he can't allow someone to antagonize him so easily. Villains will say much crueler things than Monoma did, so he can't afford to fly off the handle when lives are on the line. Deku was so overcome with fear of hurting someone that his panic prevented him from thinking of a solution. Shinso's mind control was the perfect answer, yet only Ochaco had the presence of mind to suggest it.
Blackwhip is perfect for restraining a target, so it's certainly a move Deku should prioritize mastering. The Predecessor responsible for this Quirk tells him that he can't use this power purely in anger, though it's fine to be angry as long as he controls his heart. Anger needs to sharpen Midoriya's focus, something he could certainly learn from Bakugo.
Now that Midoriya knows he is liable to burst with a new Quirk when experiencing intense emotions, learning to control himself is more important than ever. As the incident in My Hero Academia's latest Joint Training Battle showed, Deku's power is incredibly dangerous if it gets away from him. Fortunately, one of the Predecessors was there to show him the way.
If Deku wants to be the #1 Pro Hero, he must maintain his composure under emotional distress. Learning to harness and weaponize anger in a responsible way would allow him to wield more power without endangering bystanders. With new power comes new responsibility, and Deku has learned the hard way that controlling himself is just as important as controlling his Quirks.