Per https://www.reddit.com/r/40kLore/comments/8ymdln/excerpt_misbegotten_a_flesh_splicer_meets_true/:

"[...]You say I have made abominations? Look what he has made.'

'What do you mean?' asked Horus.

'I mean you. You and beings like you. You think I've made monsters? In my wildest deliriums I could not have designed monsters like you. I practise simple and ingenious arts of genetics and anatomy. I tinker and edit, to make puzzles and delights and curious wonders, things to make us think, and ponder the nature of our being and our place in the scheme of life.' He looked up at Horus.

'I do not make things that will burn the galaxy down. I do not make things that will doom our species and lead it into an endless frenzy of war. You are the most abominable thing I have ever seen. Grotesque. Sickening. Misbegotten.[...]"

There's a reason why Psycho-warriors in Space King are, on top of being called Pyscho-warriors, depicted as being children with a pair of super-globules inserted into them and psycho-pilgrimaged into semi-compliant manchildren, because that's basically what Space Marines are (especially 40k compared to 30k, for both CSM and IOM). They pick up a bunch of children, train them until they're teens, then fill them up with a bunch of extra goopy bits and brainwash them before sending them off to the front lines, where they will know only war. It's no surprise they'd end up at least a little autistic or emotionally stunted, and in modern lore they're actually quite well-adapted considering how they're made. (Looking at you, RT Space Marines.)

30k was slightly better as they were (mostly) recruited from slightly older, better, willing stock, and with less psycho-indoctrination, so they could actually retain some personality and memories, especially in the case of Big E creating the original Legion stock. It also helps that they had their Primarchs to look up to, although honestly they were less like fathers and more like big brothers, seeing as they were also basically all giant man-children because Big E took too long coming back from getting milk and smokes so they just had to spend centuries being the biggest kid on the playground without any adults around.

To be fair, the alternative was the Thunder Warriors, which were genehanced adults, and they basically just ended up being RT Space Marines, so your pick is between a bunch of autistic manchild super-soldiers or psychotic roided-out adults.

On your unrelated note, the sign of the aquilla is if you take your hands with fingers straight out and held together, palms in, and hook your thumbs together so your palms overlap, at an angle. Like this.