Sinclair sighs, watching Jack stare down the cracking tiles. It's understandable that he wouldn't know Ryan's motives, Sinclair figures he might actually have a better handle on that part than Jack. But for the short time Jack has been here, Sinclair also doubts that he himself would have motive enough to commit this murder for the reasons anyone else would. As brave as Jack is, it's hard to believe he fell from the sky purely to be Rapture's savior, defeating the tyrant to free the city. There's something else here.

He makes sure to soften his tone a great deal before he says anything else.

"So why'd you do it, son?" Simple enough, non-accusing. Just asking, if your motive wasn't Ryan's then...what was it?