Some guesswork here, but I don’t think I’m too far off:
Verst hii wolde ete & drinke..& suþþe þe louerd of þe hous quelle..& suþþe brenne al is hous al uor hor scot ywis.
First he would eat and drink...& then kill the lord of the house...& then burn all his house, all for their scot, indeed.
“Ywis” or “iwis” was used to mean “certainly, assuredly, indeed, truly. (Often with weakened sense as a metrical tag.)” according ot the OED. A modern idiomatic rendering might even be “you know.”
The gist as I see it is that the person referred to (whom I think, from a cursory look over the preceding text, is William the Conqueror, or William Bastard as the chronicler calls him) would slay his host and burn his house in payment for the food and drink.
You may be wondering about the use of the plural ("their scot"). The previous line, not quoted in the OED is “Wanne at an gode monnes house is men were at inne” (When at a good man’s house, his men were at inn [=lodged]...). So the scot to be paid is not just his, but his men’s.