I would say Erik is more wronged than bitter: more vengeful and righteous. Bitterness is such a petty emotion, and his pain was initially driven by love: love of his family and his people (which isn’t a petty motivation at all). I think he’s better and grander than that, and more (rigidly uncompromisingly) idealistic.
I don’t think he’s actually mean, either. He’s got a black/cynical sense of humour, but he’s not Loki. He doesn’t commit acts of violence maliciously, he does it because he believes the violence is necessary and morally-justified. In many ways, Erik is kind of a zealot!
The fact that he loves and admires Charles, however, shows that he does, to some extent, revere the qualities Charles has, which are the best of humanity: kindness, compassion, hope, brotherly love, etc.
Not to put words in original-Anon’s mouth, but I think there’s a very great and understandable temptation to treat Erik as the ultimate woobie, because his hurt is so vast and so deeply-buried.
Maybe that was the ‘softness’ to which they were referring — or maybe it was more Fassy they were thinking of (as in Fassy IRL, as a person) rather than Erik?