Israel Zinberg, A History of Jewish Literature, vol. 7, p. 164, quotes R. Isaac ben Elyakim’s (17th century) Yiddish book Lev Tov, which indeed does give license to kill transgressors:
For the sin of lying with a gentile woman is more grievous than adultery with a Jewess, and anyone who finds a man lying with a gentile woman may freely kill him. Indeed, it is a great mitzvah to slay him immediately, and it is like bringing him as a sacrifice, as Pinehas did. There is no need to give him any warning. Also, there is no need to obtain permission from any rabbi or from any judge or from any leader of the community, but whoever finds him . . . should kill him at once. One who slays him without judgment and without legal proceedings does a great mitzvah.
