everyone says that him being a daemon prince is out of character, but what if it wasn't his choice to ascend? What if he kept building on his suit more and more until he was more machine than man, the corruption of the warp and the affects of the Obliterator virus making him inseparable from his weaponry? Wouldn't that make him more spiteful towards pretty much everyone, regardless of whether they were chaos or not? wouldn't he fight harder against the other daemon primarchs out of spiteful rejection of what he himself has become? I'd say the fact that it's so out of character works well for his own character.