In Venom Let There Be Carnage (2021), Eddie says to Venom:
Eddie: Buddy, I'm the only person that took you in when your friends kicked you off the planet Ming-Mong, 'cause you are a reject! You are a pariah! "Lethal Protection", my ass, you couldn't protect anything!
What does "lethal protection" mean?
In purely a literal sense, 'lethal protection' -- in the context of this film -- alludes to the Venom symbiote being happy to kill some people (i.e. those it judges to be 'bad') in order to protect others (i.e. those it judges to be 'good').
See 'lethal:'
of, relating to, or causing death; deadly; fatal
See 'protection:'
the act of protecting or the state of being protected; preservation from injury or harm
In-universe, it's also a callback to an earlier line spoken by the Venom symbiote, when Eddie prevented it from biting Detective Mulligan's head off.
VENOM SYMBIOTE: Take your tiny hands off of me! We should be out there protecting the city, lethally!
Venom: Let There Be Carnage (2021)
Out-of-universe, it's a callback to the subtitle of Venom's first comicbook miniseries, Venom: Lethal Protector (1993).
Venom: Lethal Protector is a six-issue comic book limited series featuring the Eddie Brock Venom, published by Marvel Comics from February 1993 to July 1993. It was written by David Michelinie and illustrated mostly by Mark Bagley, although Ron Lim and series inker Sam DeLaRosa pencilled the last three issues.
This marked the first time Eddie Brock received a series in which he was the main character, and showed Venom's progression from a villain to an antihero. Lethal Protector would serve as the first limited series to explore Eddie Brock's background before he became Venom.
