In the Bible: The Givoni tribe tricked Joshua into a pact of non-aggression when the Jews first entered Canaan. Despite the fraud, the Jews committed to honor their agreement, and supported the Givonim by giving them a role within the Jewish community.
The book of Shemuel I (chapter 21) records that Gd punished the Jews with a famine because King Shaul later attacked the tribe of Givonim. [The commentators are perplexed, for such an attack does not appear in Tanach; some suggest this refers to his eradication of the city of Nov, which eliminated their livelihood.] The Givonim demanded a horrific price for their satisfaction: The death of seven members of King Shaul's family.
As explained in Yevamot 79a, King David reacted by declaring that no Jew could marry a member of this Givoni tribe; they could not possibly be Jewish, because they lacked the traits of bashfulness, mercy and kindness. Expanding upon this, the Rambam wrote in his work of law, Mishneh Torah (Hilchos Issurei Biah 19:17):
כל מי שיש בו עזות פנים או אכזריות ושונא את הבריות ואינו גומל להם חסד חוששין לו ביותר שמא גבעוני הוא, שסימני ישראל האומה הקדושה ביישנין רחמנים וגומלי חסדים, ובגבעונים הוא אומר והגבעונים לא מבני ישראל המה לפי שהעיזו פניהם ולא נתפייסו ולא רחמו על בני שאול ולא גמלו לישראל חסד למחול לבני מלכם והם עשו עמהם חסד והחיום בתחלהRegarding anyone who is boldfaced or cruel or who hates Gd's creatures and does not perform kindness to them: We are exceptionally concerned that this person might be a Givoni, for the signs of Israel, the holy nation, is that they are bashful, merciful and providers of kindness. Regarding the Givonim it is written, 'The Givonim were not of the children of Israel,' because they were boldfaced, and they were not appeased, and they did not have mercy upon Shaul's family, and they did not perform kindness to Israel to forgive their royal family even though the Jews had [earlier] performed kindness for them, giving them life.
The Shulchan Aruch quotes this as law in Even haEzer 2:2-
מי שיש בו עזות פנים ואכזריות ושונא את הבריות ואינו גומל להם חסד, חוששים לו ביותר שמא גבעוני הוא.Regarding anyone who is boldfaced and cruel and who hates Gd's creatures and does not perform kindness to them: We are exceptionally concerned that this person might be a Givoni.
Given the above, I have to wonder: How could anyone marry one of Beit Shemesh's self-styled Sikrikim, who - in an organized, premeditated fashion - brutalize children verbally and physically in an attempt to intimidate them and drive them out?
I know their justifications, but the Givonim had a justification, too, and it was a lot stronger than that of the Sikrikim. Claiming the banner of Torah does not hide עזות פנים (boldfaced conduct), does not mitigate אכזריות (cruelty) and does not substitute for חסד (kindness). It appears to me that we have here a modern version of חוששין לו ביותר שמא גבעוני הוא.