Unlike other comic book superheroes, Spawn's costume is so dangerous that it will kill anybody else who tries to wear it. While other companies like DC or Marvel have seen others continue the legacy of heroes, most of those characters don't get their powers from Hell. Spawn's costume is actually a living entity. The man formerly known as Al Simmons learned early on that its lethality doesn't extend just to his enemies as it tried to kill a friend simply for trying on his mask in good fun.
When Al Simmons returned to Earth after being dead for 5 years, he emerged as a Hellspawn, a leader in the demon Malebolgia's army. He returned with his body and face horribly burned and distorted, its unsettling appearance hidden by a black costume along with a billowing red cape and chains. Even Spawn's cape is stronger than most realize. While Al struggled to understand not only his new powers but the gaps in his broken memory, he also had to contend with a gauntlet of dangerous enemies ranging from demons to killers and even angels. In that time, he learned his body was comprised of necroplasmic energy and the energy used by his powers was not infinite, that he would return to Hell once it had been depleted.
In Spawn # 12 by writer and artist Todd McFarlane, Spawn and his fellow homeless enjoyed some good company along with drinking, jokes, and some singing. Considering he survived a recent gang war and a tearful reunion with a familiar face, Al let down his guard and enjoyed the comradery of those who despite having very little, accepted him as one of their own. After relieving himself behind some trashcans, one of those homeless called Gareb sees Al's discarded mask and puts it on, goofing off to the amusement of others. But to everyone's surprise, including Gareb and Simmons, the mask comes alive and attempts to suffocate its current occupant.
In comics, some identities have become iconic but over time, other people have inherited the costume or mantle when their predecessors are indisposed. Whether it's old age, injuries, or an unexplained disappearance, these characters live on but few of them had to deal with a living, sentient costume. While Venom has recently taken inspiration from Spawn, it was originally the other way around. Spawn's costume was later revealed to be a K-7 Leetha, a necroplasmic entity that, similar to Marvel's Venom symbiote, seeks to protect and empower its host. But while Venom's origins were extraterrestrial, Spawn's costume comes from Hell itself with its purpose attuned to its user and apparently it views any foreign bodies that attempt to wear or use it as a threat.
Atlhough Gareb only had it on for a few moments, Spawn's mask nearly suffocated him before Al could get it off to which the latter apologized profusely, citing he had no idea such a thing would happen. Although Gareb shook off the near-death experience with humor, this example was one of the first examples that Spawn's danger did not end with simply him. While he eventually learned how to better understand and control his powers, (Spawn even gained Superman's powers for a time) it did not minimize the overall threat Spawn represented as even his costume was not to be underestimated, easily provoked into violence and ready to attack even those considered allies if deemed dangerous.