LoS in assault phase question

As in the description pretty much.
Had one model in CC, enemy was two HQs side by side and a (multi wound) spawn to the rear of them (all in 1 unit)

I cause 1 wound, he took on Lord (who I couldn't insta kill),  passed his LoS roll and gave the wound to the spawn (who I also couldn't insta kill) who was behind him.
It annoyed me coz the other HQ that was side by side with the one I wounded could've been insta killed.

Probably a but whingey of me, but it dragged the combat out and tied up my unit for a few more turns. And it definitely could've been more useful else where

Doesn't it have to be given to the nearest model to the character who made the los?

The rule book says that it has to be the closest model to the attacked character, but not necessarily the closest to the attacker. Only when two or more other models are equally close to the attacked character, the controling player may choose which one takes the hit.

So if the Spawn was just as close to the you attacked as the other, putting the wound on it was legal.