Carrying a knife is legal for self defense as long as the knife you're carrying is a legal knife.
Some types of knives are illegal to carry so you have to check with your local state laws about the specific type of knife you're carrying.
For example a dagger knife would be illegal to carry around.
However a pocket knife or other knife that is easy to obtain and legal to own is okay and legal to carry on you and if needed you can legally use it for self defense if someone is actually trying to harm you or kill you.
If you feel your life is in danger and you need to stab someone with the knife to protect yourself then you can do so.
But if you have the option of just leaving then do that instead of stabbing someone.
While it's entirely legal to carry a pocket knife for self-defense, using it recklessly is illegal, and can lead to serious legal problems.
And as such, you should avoid drawing and using the knife unless it is absolutely necessary.
Knives are considered deadly force weapons by the courts; therefore, they should only be used in situations where the individual reasonably believes he or she is in danger of great bodily harm or death, or to protect another from the same.
Great bodily harm is a legal term that seems to be hard to define, but can generally be described as an injury that results in permanent disfigurement or loss of function.”