> Þóttust þeir Karlsefni það skilja að þessir menn myndu hafa verið
gervir brott af landinu.

"K. and his companions though that they could tell that these men must
have been made to leave the country." ("...driven from the land.")

'gervir' is the masc.nom.pl. form of the adjective variously spelt
'gørr', 'gerr', 'gjörr' which functions as the past participle of
'gøra', 'gera', 'gjöra' "to do, make", and has -v- in oblique forms
when the inflectional ending begins with a vowel. Here it agrees with
'þessir menn'.

Are they thinking that if the men are outlaws, they can safely kill
them without the locals coming for vengeance?

The preterite subjunctive 'myndu' = 'myndi' in earlier Old Norse.