Shigaraki it has always been supported, from the beginning of the story, by All for One. Gradually he then found other allies who helped him to establish a real criminal organization known as the League of Villain. Now that he has gained new powers in My Hero Academia, he hasn’t failed to show them off.
With the achievement of All for One, transferred to him by his master, Tomura Shigaraki is taking advantage of his new quirks but is also following the wishes of his master by heading to capture the One for All in the possession of Izuku Midoriya. However, there are different situations in which All for One he mentally tells him to capture the quirk that once belonged to the brother of the largest villain in history.
Shigaraki though in My Hero Academia 277 rebels against the will of his master, revealing that that is his body and therefore will not become a mere puppet of this power. Indeed, Shigaraki increases the dose by specifying that he will surpass his master and thus becoming the largest villain in existence, especially without submitting to his will. Shigaraki therefore has very clear ideas on keeping possession of his body and mind, without scruples in the betray your teacher.
How this relationship will evolve in the next chapters of My Hero Academia, e what Deku and the others can do to stop the villain? My Hero Academia may return in two weeks due to an unexpected break.