When you risk your life is a judgement call. For me it really comes down to being able to look myself in the mirror when I’m 80. Can I live with myself for not jumping in? Can I live with myself if it does have a lot of legal fallout- incarceration, bankruptcy?
In any situation, how realistic that you’ll be successful? Obviously, not wise to risk your life without a pretty significant chance of success, getting killed along with someone doesn’t help anyone. And you never know how far the other person is willing to take it, how skilled they are, whether they have friends in the area. So, like life insurance, fire extinguishers, car insurance, safety gear like motorcycle helmets, smoke detectors there are some steps to improve your odds for these low probability situations.
If you live in a jurisdiction that will issue you a Concealed Carry Weapon permit (i.e. a shall-issue state like Texas, Florida, Virgina etc). than pick a firearm, get training, get a permit.
If you’re in a jurisdiction that prefers their citizens to be unarmed victims (most parts of California, IIlinois, New York City, D.C. etc) than determine if there’s anything you can carry legally. I wouldn’t go jumping in and intervening with mace, pepper spray, or a knife but they can be handy in certain circumstances if you are confronted/attacked and can’t run.
Learn the laws in your jurisdiction, sometimes jumping in to assist someone can land you in a lot of legal trouble yourself. Warning shots are in and of themselves a felony in some places. Pulling a gun on someone, even if you think you’re stopping an attack or using it as a warning can get you a brandishing charge. Same with a knife. If you happen to taze or spray some assailant and they die of a heart attack or allergic reaction- involuntary manslaughter. Simply punching or grabbing someone and they fall down and hit their head and die without you actually being in a situation where potentially lethal force is legal can get you an involuntary manslaughter charge as well. And realize, manslaughter carries with it the same financial liability as killing someone in a car accident. They died, you didn’t intend for it to happen, you’re liable. It gets confusing very fast in court as to who was the aggressor at various points in time, roles may have changed, some jurisdictions don’t consider rape to be great bodily harm justifying lethal force etc. How much force is legal to use in assisting someone etc. It varies widely.
Calling the police is a good thing, but mostly they only arrive in time to clean up the mess. That’s the view of most of the officers I know. Frustrating for them, but it’s the reality. The current training for a policeman in an active shooter situation (think Columbine, Aurora, Virginia Tech is to rush in even prior to backup arriving. That’s because in those situations the shooter is killing 1 person every couple of minutes and normally kill themselves when police confront them.) Other than that, they wait for backup to give themselves the advantage of greater numbers, firepower, and options.