var bardaginn sem óðastur

superlative of óðr (oð, ott), a (2) furious, vehement

láta þá sjálfa á sjást

under sjá (7) reflexive – ef þit sjáist tveir á, if ye fight it out among yourselves I think fits the bill here.

eg veit hverjir valda

I think valda (veld, olla or volda, valdit), v (3) to cause, be the cause of suits the sense here better. Ie ‘it was their doing.’