But among those being targeted, gut feelings are not always reliable.
Mr. Willis said many mental-health patients smoke and take medication that makes them restless and unable to sleep, increasing the odds they will go out for a walk alone and at night. Some have trouble discerning real threats from imagined ones. As for those living in the rooming house, Mr. Willis said many are dealing with extreme poverty and have a distrust of police, making it hard for them to know where to turn for help.
"We're really trying to encourage people to think differently," he said.
The boarding house where Mr. Wass and two other victims lived is run by the non-profit organization Habitat Services, which has instituted a buddy system and heightened their security to protect their vulnerable residents.
PARC employees are focusing their outreach on the rooming house where the other victims have lived, trying to establish how many people are housed there and if they've been given any instructions on how to protect themselves.
Mr. Willis has asked all the residents to come to his centre and talk about their safety, although he worries many will resist any association with a facility known for treating the mentally ill.
But the threat against these groups has propelled others in the neighbourhood to step forward.
Paisley Rae, a social-media consultant who lives at King and Dufferin, became aware of the attacks through the publicity surrounding Mr. Wass's death, and attended the PARC meeting where she volunteered to help.
"I was left with the feeling of 'My god, I absolutely need to do something but I don't know what to do,'" she said. "It makes me feel bad about humanity."
Working with Toronto Police, Ms. Rae produced a PSA video that has been posted on YouTube, informing people about the crimes and appealing for community involvement.
"Somebody knows who's doing it and they're not saying anything," she said.
David Anthony Domet, who knew Mr. Wass through the Holy Family Parish on King Street West, said he worries about the community's other vulnerable populations: new immigrants, young families, single parents and children.
"He's obviously going after someone weak, an easy victim," he said. "It's just wanton violence."
The attacks have been distressing for the entire community, he said, and raise questions beyond just who is responsible.
He wants to know why the city is "growing cold" to such violence, and said the level of outrage and upset surrounding the attacks is not where it should be.
Also troubling is the fact that Mr. Wass was released from hospital, he said, only to die from his injuries days later.
"He didn't know how to take care of himself, it was obvious to anyone who met him," he said. "Everybody's wondered - what the heck was he doing out of the hospital?"
Michelle Tadique, a spokesperson for St. Joseph's, said the hospital could not release any information on Mr. Wass's treatment due to patient confidentiality, and would not say whether they were reviewing their actions.
But Mr. Willis said he has corresponded directly with Carolyn Baker, the hospital's chief executive officer, in the wake of Mr. Wass's death.
Opportunity arises from tragedy
If there is a bright side to what has happened, says Mr. Willis, it is the opportunity it has given the community to confront how it treats its most at-risk residents.
St. Joe's is one of the city's busiest emergency rooms for people with mental-health issues, he pointed out, and his organization is asking the hospital to reexamine how they deal with that demographic.
"Can they be a leader on this? Look at the population they serve - they should be," he said.
At PARC, employees have fielded calls from local artists, unions and other groups offering their services in the investigation.
Scott McKean, supervisor for the City of Toronto Community Crisis Response Program, has gotten involved, and is considering hosting a "take back the night" event.
The police, too, have made it clear they are dedicated to solving the crime, reaching out to community members, setting up a tip line and establishing a constant presence in the area. Tips have been coming in, said Constable Vella, a rarity in a neighbourhood not prone to trusting the police.
Not long ago, Mr. Willis said many in the community would have turned a blind eye to attacks on people like Mr. Wass, who they didn't consider to be their problem.
"Compared to how it would have been 15 years ago, the whole community has gotten involved," he said. "We're fighting this guy on a different battleground than it once was, and that's a positive thing."