For Mordred, see http://kingarthur.wikia.com/wiki/Mordred .
Note that there are different versions of the story of Mordred in different texts.None of them indicates exactly why Mordred hates Arthur.
In most tales, Mordred is killed by King Arthur’s forces, sometimes by King Arthur himself, but King Arthur is mortally wounded in the same battle, sometimes by Mordred himself.
Mordred is never presented as English, that is, as Anglo-Saxon. Nor is he presented as Welsh. Mordred’s father in earlier accounts is King Loth of Lothian, who is supposedly of Norwegian origin, being the nephew of King Sichelm of Norway. His mother was King Arthur’s sister in some accounts, or King Arthur’s half-sister in others. In later accounts Mordred is an illegitimate son of Arthur, sired on his sister.
No sources particularly indicates that her ancestry was Welsh. As far as it goes, it probably can be considered to be British. She was the daughter of Ygerne/Eigyr, who in Welsh sources is the daughter of one Anblaut Guletic (Amlawdd Wledig or Anlawdd Wledig), of whom we know nothing except that in tales he is given many daughters, apparently as a convenient way to genealogically link other figures who supposedly lived at the time to Arthur.
The answerer who claims Mordred is Welsh is just making it up.