(CNN) It's an instinct that comes naturally to good Samaritans: When you see one stranger attacking another with racist or religious epithets, you try to protect the victim.

But when two men did exactly that on a Portland train, they were stabbed to death

So what is a bystander to do? Should you confront the attacker and try to de-escalate the situation? Or should you avoid getting involved?

"This is a profound dilemma for the general public," said Andrew J. Scott, a retired police chief and president of AJS Consulting

There is no easy answer for every situation. But here are how a few recent cases have turned out, and what Scott recommends you consider if you do decide to intervene:

The triple stabbing in Portland

Rick Best, Taliesin Myrddin Namkai Meche and Micah Fletcher saw two girls being berated on a Portland light rail train. They didn't sit still.

A man was confronting the two teens -- including one who was wearing a hijab -- and started spouting "hate speech toward a variety of ethnicities and religions," Portland Police Sgt. Pete Simpson said.

of the experience. "He told us to go back to Saudi Arabia and he told us we shouldn't be here, to get out of his country," Destinee Mangum, 16, told CNN affiliate KPTV

When the trio of men tried to calm the suspect, all three were stabbed. Best and Meche died; Fletcher was stabbed 1 millimeter away from his jugular

Scott said it's likely difficult, if not impossible, for those good Samaritans to have predicted the the assailant would try to kill them.

"In this society, we're raised to not allow bullies to engage in racist behavior and to intervene verbally," Scott said. "Bullies tend to back down when verbally challenged."

But with the Portland incident, "Clearly this is not the case," Scott said. "(The assailant) is a sociopath or a psychopath, and it's not the norm."

The more peaceful endings

There are many more cases that don't end in violence.

On May 22, a woman at an Arkansas Walmart told another customer to " go back to Mexico ." When a black customer told the woman to stop being ignorant, the woman started calling her the N-word.

Walmart later asked the woman to stop shopping at its stores.

Sometimes, the interventions lead to praise and reward. In March, Tracey Tong confronted a woman on a New York subway who was berating another passenger she believed was Muslim.

"Why are you here? Why are you in this country?" the woman asked . "You're not with us."

Tong fired back at the instigator. "Don't start attacking an innocent woman," she said. "... I'm not telling you to be quiet. I'm asking you to please respect her."

The video was shared on Facebook more than 20,000 times, and Tong was invited on "The Ellen Degeneres" show, where she received a $10,000 reward

So what should you do?

Scott, who worked in law enforcement for three decades, said ultimately the decision on whether to intervene is a highly personal one.

But if you do want to act, Scott suggests considering the following:

-- Is the perpetrator much larger than you?

-- Does he or she appear to be intoxicated or mentally unstable?

-- If things get ugly, is there an easy escape route? Or are you stuck in a confined area?

If you still decide to intervene, "Understand it's a fluid situation that can go from bad to worse," Scott said.

"That's another dilemma that the public faces to help their fellow man or person ... these are the types of things that are occurring in a split second, and people have to make split-second decisions."