There's kind of an interesting thing about Labrador Retrievers (well, all retrievers, actually): they were bred with a purpose in mind, and bred to be obedient and friendly. (This seems to be true in the Beastars world as much as our own, based on an earlier flashback chapter with folks trying to goad Legosi and Jack into fighting, because Jack was the 'improved' version of a canine.)
JESUS CHRIST, JACK, YOU DUMB FUCK, DON'T DIE BY EATING A SACK OF ONIONS!
You were so happy to see Legosi back - how sad do you think he and the rest of room 701 would be if you offed yourself?
Unless the 'secret history' here is much worse than what we know already, or Jack's just reacting a lot more strongly to that, because he's been bred to do so (wait, did they actually do eugenics on dogs? Because I'D get really fucking mad if I found out that'd happened to my family), this doesn't make a lot of sense for Jack to pull.
I do love what Bill's doing, though. He's come a LONG way from the beginning of the manga.