If you don't actually feel threatened, I'd probably ignore it. I'd point out that pretty anyone can kill someone else, given a will to do so, so I don't think I'd take "oh, they're physical weaker than me" or something as grounds for not taking a threat seriously.

But if it's a credible threat...depending upon where you live and the form of the threat, a threat to kill someone may be illegal. They are one of the few exceptions that case law has established to the First Amendment in the US.

https://en.wikipedia.org/wiki/United_States_free_speech_exceptions

"True threats of violence" that are directed at a person or group of persons that have the intent of placing the target at risk of bodily harm or death are generally unprotected.[41] However, there are several exceptions. For example, the Supreme Court has held that "threats may not be punished if a reasonable person would understand them as obvious hyperbole", he writes.[42][43] Additionally, threats of "social ostracism" and of "politically motivated boycotts" are constitutionally protected.[44]

In California, for example:

https://law.justia.com/codes/california/code-pen/part-1/title-11-5/section-422/

CA Penal Code § 422 (2025)

422. (a) Any person who willfully threatens to commit a crime which will result in death or great bodily injury to another person, with the specific intent that the statement, made verbally, in writing, or by means of an electronic communication device, is to be taken as a threat, even if there is no intent of actually carrying it out, which, on its face and under the circumstances in which it is made, is so unequivocal, unconditional, immediate, and specific as to convey to the person threatened, a gravity of purpose and an immediate prospect of execution of the threat, and thereby causes that person reasonably to be in sustained fear for their own safety or for their immediate family’s safety, shall be punished by imprisonment in the county jail not to exceed one year, or by imprisonment in the state prison.

(b) In sentencing a person convicted of a felony violation of subdivision (a), the court may consider, as a factor in aggravation, that the defendant willfully threatened to commit a crime that would result in the death or great bodily injury of a person the defendant knew was a state constitutional officer, a Member of the Legislature, or a judge or court commissioner, as defined in subdivisions (a), (b), (c), (n), and (q) of Section 7920.500 of the Government Code.

(c) (1) For purposes of this section, “immediate family” means any spouse, whether by marriage or not, parent, child, any person related by consanguinity or affinity within the second degree, or any other person who regularly resides in the household, or who, within the prior six months, regularly resided in the household.

(2) For purposes of this section, “electronic communication device” includes, but is not limited to, telephones, cellular telephones, computers, video recorders, fax machines, or pagers. “Electronic communication” has the same meaning as the term is defined in Subsection 12 of Section 2510 of Title 18 of the United States Code.