It seems to me his whole earlier childhood was such: He was forced to sell himself to get something to eat, among other things, like killing people. His new daddy likely met him first due to such circumstances, or due to some gang connections, and ended up adopting him as a son withbenefitsdisadvantages...
Wasn't Ein the dude from Mars (or at least the Mars branch of Gjallarhorn) whom McGillis took with him, outwardly as an ally and to help him advance in the ranks, but in reality he needed a tool he could use in experiements (Ein ended up more a machine, less a man towards the end of the first season) and sacrifice. Vidar was his bro in the new family, the other brat in the flashbacks (Gaelio). He didn't view kindly slaying that fool Carta Issue, among other things. I don't actually exactly remember what happened to him at the end of the first season. Was he just allowed to flee or something?