There are a number of ways that Sula and Shadrack bear some similarities to one another.
Each character leaves Medallion and returns with a particular sense of self that functions to isolate them from the rest of the town. While Shadrack's sensibility is tied to trauma and injury directly, Sula's self-hood is also connected to trauma, having seen her mother burned to death (which is also how her uncle died).
Both Sula and Shadrack inspire the people of Medallion to express an inner violence at the end of the novel. Sula's powerful presence in the town is associated finally with a dreadful but short-lived winter that brings a thaw to the town's ability to suppress its inner nature.
While Sula remains in Medallion, the town is allowed to reactive instead of active. The people band together in response (reaction) to circumstances that Sula creates and/or represents.
Without Sula, the town comes to understand a joyous and violent sense of self that is manifested on the Suicide Day parade led by Shadrack. This parade leads to many deaths in Medallion.
The warm January thaw and the soft, water-soaked ground lead to the deaths of many Bottom residents who follow Shadrack to the New River Road tunnel to be crushed or drowned. Some are victims of the powerful forces that can overwhelm human beings while others watch. (eNotes)
There is irony built into this scenario. The people of Medallion are finally inspired to live, inspired in part by Sula's example of tapping into an inner self. By living, they quickly act in ways that bring death.
Sula and Shadrack bring people together in ways that are explicitly associated with life and death.
"On the third day of the new year, [Shadrack] walked through the Bottom down Carpenter's Road with a cowbell and a hangman's rope calling people together. Telling them that this was their only chance to kill themselves or each other."
Connected to robins and to larger forces of nature, Sula causes the people to love one another anew, tightening the bonds of relationships that had been loose. Both characters are singular and set apart but they function to bring others together.
(There is also a notable moment when Sula runs to Shadrack's house after accidentally killing Chicken Little. Shadrack and Sula share an odd moment, with Shadrack saying, "Always," and somehow calming the distressed young Sula. A thematic connection between the two characters is only insinuated here but we might see in this scene an underlying strangeness or difference that defines these two people, one that is born out of death and violence.)
On a formal level, we can also note that these two characters are each presented at the outset of the respective parts of the novel. They both return from a time away and in their return disturb and re-define the life of the town. They are aberrations, departures from the norms. From the Bottom, they are still not quite "of" the Bottom.