John Gill Commentary 1 Samuel 20:15

"but also thou shalt not cut off thy kindness from my house for ever; no, not when Jehovah hath cut off the enemies of David every one from the face of the earth." — 1 Samuel 20:15 (ASV)

But also you shall not cut off your kindness from my house
for ever
His family should partake of it as well as himself:

no, not when the Lord has cut off the enemies of David, everyone
from the face of the earth ;
Saul and his sons, and everyone that should oppose his settlement in the kingdom: Jonathan's meaning is, that the covenant between them should not be only between them personally, but include their posterity, as follows.