when the siblings were introduced it was clear they were all representatives of the seven deadly sins
Envy, Lust, Gluttony, Sloth were easily represented and the two left out are either Pride or Greed. so the real personality of Jilbagaias's would have been Wrath. and from the perspective of wrath the human reborn in him is far angrier then he could ever be so it makes complete sense why he would think over all that the 'human' has done and be in full support